[fastapi] Handle ellipsis defaults in FAST002 autofix (FAST002) (#20810)

## Summary

Implement handling of ellipsis (`...`) defaults in the `FAST002` autofix
to correctly differentiate between required and optional parameters in
FastAPI route definitions.

Previously, the autofix did not properly handle cases where parameters
used `...` as a default value (to indicate required parameters). This
could lead to incorrect transformations when applying the autofix.

This change updates the `FAST002` autofix logic to:
- Correctly recognize `...` as a valid FastAPI required default.
- Preserve the semantics of required parameters while still applying
other autofix improvements.
- Avoid incorrectly substituting or removing ellipsis defaults.

Fixes #20800

## Test Plan

Added a new test fixture at:
```crates/ruff_linter/resources/test/fixtures/fastapi/FAST002_2.py```

Author: hengky-kurniawan-1

crates/ruff_linter/resources/test/fixtures/fastapi/FAST002_2.py

@@ -0,0 +1,87 @@
+"""Test FAST002 ellipsis handling."""
+
+from fastapi import Body, Cookie, FastAPI, Header, Query
+
+app = FastAPI()
+
+
+# Cases that should be fixed - ellipsis should be removed
+
+
+@app.get("/test1")
+async def test_ellipsis_query(
+    # This should become: param: Annotated[str, Query(description="Test param")]
+    param: str = Query(..., description="Test param"),
+) -> str:
+    return param
+
+
+@app.get("/test2")
+async def test_ellipsis_header(
+    # This should become: auth: Annotated[str, Header(description="Auth header")]
+    auth: str = Header(..., description="Auth header"),
+) -> str:
+    return auth
+
+
+@app.post("/test3")
+async def test_ellipsis_body(
+    # This should become: data: Annotated[dict, Body(description="Request body")]
+    data: dict = Body(..., description="Request body"),
+) -> dict:
+    return data
+
+
+@app.get("/test4")
+async def test_ellipsis_cookie(
+    # This should become: session: Annotated[str, Cookie(description="Session ID")]
+    session: str = Cookie(..., description="Session ID"),
+) -> str:
+    return session
+
+
+@app.get("/test5")
+async def test_simple_ellipsis(
+    # This should become: id: Annotated[str, Query()]
+    id: str = Query(...),
+) -> str:
+    return id
+
+
+@app.get("/test6")
+async def test_multiple_kwargs_with_ellipsis(
+    # This should become: param: Annotated[str, Query(description="Test", min_length=1, max_length=10)]
+    param: str = Query(..., description="Test", min_length=1, max_length=10),
+) -> str:
+    return param
+
+
+# Cases with actual default values - these should preserve the default
+
+
+@app.get("/test7")
+async def test_with_default_value(
+    # This should become: param: Annotated[str, Query(description="Test")] = "default"
+    param: str = Query("default", description="Test"),
+) -> str:
+    return param
+
+
+@app.get("/test8")
+async def test_with_default_none(
+    # This should become: param: Annotated[str | None, Query(description="Test")] = None
+    param: str | None = Query(None, description="Test"),
+) -> str:
+    return param or "empty"
+
+
+@app.get("/test9")
+async def test_mixed_parameters(
+    # First param should be fixed with default preserved  
+    optional_param: str = Query("default", description="Optional"),
+    # Second param should not be fixed because of the preceding default
+    required_param: str = Query(..., description="Required"),
+    # Third param should be fixed with default preserved
+    another_optional_param: int = Query(42, description="Another optional"),
+) -> str:
+    return f"{required_param}-{optional_param}-{another_optional_param}"

crates/ruff_linter/src/rules/fastapi/mod.rs

@@ -18,6 +18,7 @@ mod tests {
     #[test_case(Rule::FastApiRedundantResponseModel, Path::new("FAST001.py"))]
     #[test_case(Rule::FastApiNonAnnotatedDependency, Path::new("FAST002_0.py"))]
     #[test_case(Rule::FastApiNonAnnotatedDependency, Path::new("FAST002_1.py"))]
+    #[test_case(Rule::FastApiNonAnnotatedDependency, Path::new("FAST002_2.py"))]
     #[test_case(Rule::FastApiUnusedPathParameter, Path::new("FAST003.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.name(), path.to_string_lossy());
@@ -56,6 +57,7 @@ mod tests {
     // since `typing.Annotated` was added in Python 3.9
     #[test_case(Rule::FastApiNonAnnotatedDependency, Path::new("FAST002_0.py"))]
     #[test_case(Rule::FastApiNonAnnotatedDependency, Path::new("FAST002_1.py"))]
+    #[test_case(Rule::FastApiNonAnnotatedDependency, Path::new("FAST002_2.py"))]
     fn rules_py38(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}_py38", rule_code.name(), path.to_string_lossy());
         let diagnostics = test_path(

crates/ruff_linter/src/rules/fastapi/rules/fastapi_non_annotated_dependency.rs

@@ -275,19 +275,42 @@ fn create_diagnostic(
                     .collect::<Vec<_>>()
                     .join(", ");
 
-                seen_default = true;
-                format!(
-                    "{parameter_name}: {binding}[{annotation}, {default_}({kwarg_list})] \
-                            = {default_value}",
+                // Check if the default argument is ellipsis (...), which in FastAPI means "required"
+                let is_default_argument_ellipsis = matches!(
+                    dependency_call.default_argument.value(),
+                    ast::Expr::EllipsisLiteral(_)
+                );
+
+                if is_default_argument_ellipsis && seen_default {
+                    // For ellipsis after a parameter with default, can't remove the default
+                    return Ok(None);
+                }
+
+                if !is_default_argument_ellipsis {
+                    // For actual default values, mark that we've seen a default
+                    seen_default = true;
+                }
+
+                let base_format = format!(
+                    "{parameter_name}: {binding}[{annotation}, {default_}({kwarg_list})]",
                     parameter_name = parameter.name,
                     annotation = checker.locator().slice(parameter.annotation.range()),
                     default_ = checker
                         .locator()
                         .slice(map_callable(parameter.default).range()),
-                    default_value = checker
+                );
+
+                if is_default_argument_ellipsis {
+                    // For ellipsis, don't add a default value since the parameter
+                    // should remain required after conversion to Annotated
+                    base_format
+                } else {
+                    // For actual default values, preserve them
+                    let default_value = checker
                         .locator()
-                        .slice(dependency_call.default_argument.value().range()),
-                )
+                        .slice(dependency_call.default_argument.value().range());
+                    format!("{base_format} = {default_value}")
+                }
             }
             _ => {
                 if seen_default {