[ty] Extend Final test suite (#19476)

## Summary

Restructures and cleans up the `typing.Final` test suite. Also adds a
few more tests with TODOs based on the [typing spec for
`typing.Final`](https://typing.python.org/en/latest/spec/qualifiers.html#uppercase-final).

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/type_qualifiers/final.md

@@ -19,10 +19,6 @@ FINAL_A: Final[int] = 1
 FINAL_B: Annotated[Final[int], "the annotation for FINAL_B"] = 1
 FINAL_C: Final[Annotated[int, "the annotation for FINAL_C"]] = 1
 FINAL_D: "Final[int]" = 1
-# Note: Some type checkers do not support a separate declaration and
-# assignment for `Final` symbols, but it's possible to support this in
-# ty, and is useful for code that declares symbols `Final` inside
-# `if TYPE_CHECKING` blocks.
 FINAL_F: Final[int]
 FINAL_F = 1
 
@@ -52,7 +48,7 @@ reveal_type(FINAL_D)  # revealed: int
 reveal_type(FINAL_F)  # revealed: int
 ```
 
-### `Final` without a type
+### Bare `Final` without a type
 
 When a symbol is qualified with `Final` but no type is specified, the type is inferred from the
 right-hand side of the assignment. We do not union the inferred type with `Unknown`, because the
@@ -231,7 +227,96 @@ FINAL_LIST: Final[list[int]] = [1, 2, 3]
 FINAL_LIST[0] = 4
 ```
 
-## Too many arguments
+## Overriding in subclasses
+
+When a symbol is qualified with `Final` in a class, it cannot be overridden in subclasses.
+
+```py
+from typing import Final
+
+class Base:
+    FINAL_A: Final[int] = 1
+    FINAL_B: Final[int] = 1
+    FINAL_C: Final = 1
+
+class Derived(Base):
+    # TODO: This should be an error
+    FINAL_A = 2
+    # TODO: This should be an error
+    FINAL_B: Final[int] = 2
+    # TODO: This should be an error
+    FINAL_C = 2
+```
+
+## Syntax and usage
+
+### Legal syntactical positions
+
+Final may only be used in assignments or variable annotations. Using it in any other position is an
+error.
+
+```py
+from typing import Final, ClassVar, Annotated
+
+LEGAL_A: Final[int] = 1
+LEGAL_B: Final = 1
+LEGAL_C: Final[int]
+LEGAL_C = 1
+LEGAL_D: Final
+LEGAL_D = 1
+
+class C:
+    LEGAL_E: ClassVar[Final[int]] = 1
+    LEGAL_F: Final[ClassVar[int]] = 1
+    LEGAL_G: Annotated[Final[ClassVar[int]], "metadata"] = 1
+
+    def __init__(self):
+        self.LEGAL_H: Final[int] = 1
+        self.LEGAL_I: Final[int]
+        self.LEGAL_I = 1
+
+# TODO: This should be an error
+def f(ILLEGAL: Final[int]) -> None:
+    pass
+
+# TODO: This should be an error
+def f() -> Final[None]: ...
+
+# TODO: This should be an error
+class Foo(Final[tuple[int]]): ...
+
+# TODO: Show `Unknown` instead of `@Todo` type in the MRO; or ignore `Final` and show the MRO as if `Final` was not there
+# revealed: tuple[<class 'Foo'>, @Todo(Inference of subscript on special form), <class 'object'>]
+reveal_type(Foo.__mro__)
+```
+
+### Attribute assignment outside `__init__`
+
+Qualifying an instance attribute with `Final` outside of `__init__` is not allowed. The instance
+attribute must be assigned only once, when the instance is created.
+
+```py
+from typing import Final
+
+class C:
+    def some_method(self):
+        # TODO: This should be an error
+        self.x: Final[int] = 1
+```
+
+### `Final` in loops
+
+Using `Final` in a loop is not allowed.
+
+```py
+from typing import Final
+
+for _ in range(10):
+    # TODO: This should be an error
+    i: Final[int] = 1
+```
+
+### Too many arguments
 
 ```py
 from typing import Final
@@ -241,39 +326,58 @@ class C:
     x: Final[int, str] = 1
 ```
 
-## Illegal `Final` in type expression
+### Illegal `Final` in type expression
 
 ```py
 from typing import Final
 
+# error: [invalid-type-form] "Type qualifier `typing.Final` is not allowed in type expressions (only in annotation expressions)"
+x: list[Final[int]] = []  # Error!
+
 class C:
-    # error: [invalid-type-form] "Type qualifier `typing.Final` is not allowed in type expressions (only in annotation expressions)"
+    # error: [invalid-type-form]
     x: Final | int
 
-    # error: [invalid-type-form] "Type qualifier `typing.Final` is not allowed in type expressions (only in annotation expressions)"
+    # error: [invalid-type-form]
     y: int | Final[str]
 ```
 
 ## No assignment
 
+Some type checkers do not support a separate declaration and assignment for `Final` symbols, but
+it's possible to support this in ty, and is useful for code that declares symbols `Final` inside
+`if TYPE_CHECKING` blocks.
+
+### Basic
+
 ```py
 from typing import Final
 
 DECLARED_THEN_BOUND: Final[int]
 DECLARED_THEN_BOUND = 1
 ```
 
-## No assignment for bare `Final`
+### No assignment
 
 ```py
 from typing import Final
 
 # TODO: This should be an error
-NO_RHS: Final
+NO_ASSIGNMENT_A: Final
+# TODO: This should be an error
+NO_ASSIGNMENT_B: Final[int]
 
 class C:
     # TODO: This should be an error
-    NO_RHS: Final
+    NO_ASSIGNMENT_A: Final
+    # TODO: This should be an error
+    NO_ASSIGNMENT_B: Final[int]
+
+    # This is okay. `DEFINED_IN_INIT` is defined in `__init__`.
+    DEFINED_IN_INIT: Final[int]
+
+    def __init__(self):
+        self.DEFINED_IN_INIT = 1
 ```
 
 ## Full diagnostics