[ty] Improve tests for site-packages discovery (#18374)

## Summary

- Convert tests demonstrating our resilience to malformed/absent
`version` fields in `pyvenf.cfg` files to mdtests. Also make them more
expansive.
- Convert the regression test I added in
#18157 to an mdtest
- Add comments next to unit tests that cannot be converted to mdtests
(but where it's not obvious why they can't) so I don't have to do this
exercise again 😄
- In `site_packages.rs`, factor out the logic for figuring out where we
expect the system-installation `site-packages` to be. Currently we have
the same logic twice.

## Test Plan

`cargo test -p ty_python_semantic`

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/import/site_packages_discovery.md

@@ -1,5 +1,115 @@
 # Tests for `site-packages` discovery
 
+## Malformed or absent `version` fields
+
+The `version`/`version_info` key in a `pyvenv.cfg` file is provided by most virtual-environment
+creation tools to indicate the Python version the virtual environment is for. They key is useful for
+our purposes, so we try to parse it when possible. However, the key is not read by the CPython
+standard library, and is provided under different keys depending on which virtual-environment
+creation tool created the `pyvenv.cfg` file (the stdlib `venv` module calls the key `version`,
+whereas uv and virtualenv both call it `version_info`). We therefore do not return an error when
+discovering a virtual environment's `site-packages` directory if the virtula environment contains a
+`pyvenv.cfg` file which doesn't have this key, or if the associated value of the key doesn't parse
+according to our expectations. The file isn't really *invalid* in this situation.
+
+### No `version` field
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+
+reveal_type(X)  # revealed: int
+```
+
+### Malformed stdlib-style version field
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+version = wut
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+
+reveal_type(X)  # revealed: int
+```
+
+### Malformed uv-style version field
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+version_info = no-really-wut
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+
+reveal_type(X)  # revealed: int
+```
+
 ## Ephemeral uv environments
 
 If you use the `--with` flag when invoking `uv run`, uv will create an "ephemeral" virtual
@@ -57,3 +167,41 @@ from bar import Y
 reveal_type(X)  # revealed: int
 reveal_type(Y)  # revealed: str
 ```
+
+## `pyvenv.cfg` files with unusual values
+
+`pyvenv.cfg` files can have unusual values in them, which can contain arbitrary characters. This
+includes `=` characters. The following is a regression test for
+<https://github.com/astral-sh/ty/issues/430>.
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+version_info = 3.13
+command = /.pyenv/versions/3.13.3/bin/python3.13 -m venv --without-pip --prompt="python-default/3.13.3" /somewhere-else/python/virtualenvs/python-default/3.13.3
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+
+reveal_type(X)  # revealed: int
+```

crates/ty_python_semantic/src/site_packages.rs

@@ -1001,22 +1001,7 @@ mod tests {
                 ))
             };
 
-            let expected_system_site_packages = if cfg!(target_os = "windows") {
-                SystemPathBuf::from(&*format!(
-                    r"\Python3.{}\Lib\site-packages",
-                    self.minor_version
-                ))
-            } else if self.free_threaded {
-                SystemPathBuf::from(&*format!(
-                    "/Python3.{minor_version}/lib/python3.{minor_version}t/site-packages",
-                    minor_version = self.minor_version
-                ))
-            } else {
-                SystemPathBuf::from(&*format!(
-                    "/Python3.{minor_version}/lib/python3.{minor_version}/site-packages",
-                    minor_version = self.minor_version
-                ))
-            };
+            let expected_system_site_packages = self.expected_system_site_packages();
 
             if self_venv.system_site_packages {
                 assert_eq!(
@@ -1051,33 +1036,33 @@ mod tests {
             );
 
             let site_packages_directories = env.site_packages_directories(&self.system).unwrap();
+            let expected_site_packages = self.expected_system_site_packages();
+            assert_eq!(
+                site_packages_directories,
+                std::slice::from_ref(&expected_site_packages)
+            );
+        }
 
-            let expected_site_packages = if cfg!(target_os = "windows") {
-                SystemPathBuf::from(&*format!(
-                    r"\Python3.{}\Lib\site-packages",
-                    self.minor_version
-                ))
+        fn expected_system_site_packages(&self) -> SystemPathBuf {
+            let minor_version = self.minor_version;
+            if cfg!(target_os = "windows") {
+                SystemPathBuf::from(&*format!(r"\Python3.{minor_version}\Lib\site-packages"))
             } else if self.free_threaded {
                 SystemPathBuf::from(&*format!(
-                    "/Python3.{minor_version}/lib/python3.{minor_version}t/site-packages",
-                    minor_version = self.minor_version
+                    "/Python3.{minor_version}/lib/python3.{minor_version}t/site-packages"
                 ))
             } else {
                 SystemPathBuf::from(&*format!(
-                    "/Python3.{minor_version}/lib/python3.{minor_version}/site-packages",
-                    minor_version = self.minor_version
+                    "/Python3.{minor_version}/lib/python3.{minor_version}/site-packages"
                 ))
-            };
-
-            assert_eq!(
-                site_packages_directories,
-                [expected_site_packages].as_slice()
-            );
+            }
         }
     }
 
     #[test]
     fn can_find_site_packages_directory_no_virtual_env() {
+        // Shouldn't be converted to an mdtest because mdtest automatically creates a
+        // pyvenv.cfg file for you if it sees you creating a `site-packages` directory.
         let test = PythonEnvironmentTestCase {
             system: TestSystem::default(),
             minor_version: 12,
@@ -1090,6 +1075,8 @@ mod tests {
 
     #[test]
     fn can_find_site_packages_directory_no_virtual_env_freethreaded() {
+        // Shouldn't be converted to an mdtest because mdtest automatically creates a
+        // pyvenv.cfg file for you if it sees you creating a `site-packages` directory.
         let test = PythonEnvironmentTestCase {
             system: TestSystem::default(),
             minor_version: 13,
@@ -1132,23 +1119,10 @@ mod tests {
         );
     }
 
-    #[test]
-    fn can_find_site_packages_directory_no_version_field_in_pyvenv_cfg() {
-        let test = PythonEnvironmentTestCase {
-            system: TestSystem::default(),
-            minor_version: 12,
-            free_threaded: false,
-            origin: SysPrefixPathOrigin::VirtualEnvVar,
-            virtual_env: Some(VirtualEnvironmentTestCase {
-                pyvenv_cfg_version_field: None,
-                ..VirtualEnvironmentTestCase::default()
-            }),
-        };
-        test.run();
-    }
-
     #[test]
     fn can_find_site_packages_directory_venv_style_version_field_in_pyvenv_cfg() {
+        // Shouldn't be converted to an mdtest because we want to assert
+        // that we parsed the `version` field correctly in `test.run()`.
         let test = PythonEnvironmentTestCase {
             system: TestSystem::default(),
             minor_version: 12,
@@ -1164,6 +1138,8 @@ mod tests {
 
     #[test]
     fn can_find_site_packages_directory_uv_style_version_field_in_pyvenv_cfg() {
+        // Shouldn't be converted to an mdtest because we want to assert
+        // that we parsed the `version` field correctly in `test.run()`.
         let test = PythonEnvironmentTestCase {
             system: TestSystem::default(),
             minor_version: 12,
@@ -1179,6 +1155,8 @@ mod tests {
 
     #[test]
     fn can_find_site_packages_directory_virtualenv_style_version_field_in_pyvenv_cfg() {
+        // Shouldn't be converted to an mdtest because we want to assert
+        // that we parsed the `version` field correctly in `test.run()`.
         let test = PythonEnvironmentTestCase {
             system: TestSystem::default(),
             minor_version: 12,
@@ -1209,6 +1187,9 @@ mod tests {
 
     #[test]
     fn finds_system_site_packages() {
+        // Can't be converted to an mdtest because the system installation's `sys.prefix`
+        // path is at a different location relative to the `pyvenv.cfg` file's `home` value
+        // on Windows.
         let test = PythonEnvironmentTestCase {
             system: TestSystem::default(),
             minor_version: 13,
@@ -1366,25 +1347,6 @@ mod tests {
         );
     }
 
-    /// See <https://github.com/astral-sh/ty/issues/430>
-    #[test]
-    fn parsing_pyvenv_cfg_with_equals_in_value() {
-        let test = PythonEnvironmentTestCase {
-            system: TestSystem::default(),
-            minor_version: 13,
-            free_threaded: true,
-            origin: SysPrefixPathOrigin::VirtualEnvVar,
-            virtual_env: Some(VirtualEnvironmentTestCase {
-                pyvenv_cfg_version_field: Some("version_info = 3.13"),
-                command_field: Some(
-                    r#"command = /.pyenv/versions/3.13.3/bin/python3.13 -m venv --without-pip --prompt="python-default/3.13.3" /somewhere-else/python/virtualenvs/python-default/3.13.3"#,
-                ),
-                ..VirtualEnvironmentTestCase::default()
-            }),
-        };
-        test.run();
-    }
-
     #[test]
     fn parsing_pyvenv_cfg_with_key_but_no_value_fails() {
         let system = TestSystem::default();