[fastapi] Fix false positive for paths with spaces around parameters (FAST003) (#20077)

## Summary

Fixes #20060

crates/ruff_linter/resources/test/fixtures/fastapi/FAST003.py

@@ -213,3 +213,17 @@ async def get_id_pydantic_full(
 async def get_id_pydantic_short(params: Annotated[PydanticParams, Depends()]): ...
 @app.get("/{my_id}")
 async def get_id_init_not_annotated(params = Depends(InitParams)): ...
+
+@app.get("/things/{ thing_id }")
+async def read_thing(query: str):
+    return {"query": query}
+
+
+@app.get("/things/{ thing_id : path }")
+async def read_thing(query: str):
+    return {"query": query}
+
+
+@app.get("/things/{ thing_id : str }")
+async def read_thing(query: str):
+    return {"query": query}

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

@@ -457,6 +457,9 @@ fn parameter_alias<'a>(parameter: &'a Parameter, semantic: &SemanticModel) -> Op
 ///
 /// The iterator yields tuples of the parameter name and the range of the parameter in the input,
 /// inclusive of curly braces.
+///
+/// FastAPI only recognizes path parameters when there are no leading or trailing spaces around
+/// the parameter name. For example, `/{x}` is a valid parameter, but `/{ x }` is treated literally.
 #[derive(Debug)]
 struct PathParamIterator<'a> {
     input: &'a str,
@@ -483,7 +486,7 @@ impl<'a> Iterator for PathParamIterator<'a> {
                     // We ignore text after a colon, since those are path converters
                     // See also: https://fastapi.tiangolo.com/tutorial/path-params/?h=path#path-convertor
                     let param_name_end = param_content.find(':').unwrap_or(param_content.len());
-                    let param_name = &param_content[..param_name_end].trim();
+                    let param_name = &param_content[..param_name_end];
 
                     #[expect(clippy::range_plus_one)]
                     return Some((param_name, start..end + 1));

crates/ruff_linter/src/rules/fastapi/snapshots/ruff_linter__rules__fastapi__tests__fast-api-unused-path-parameter_FAST003.py.snap

@@ -59,25 +59,6 @@ help: Add `thing_id` to function signature
 23 | 
 note: This is an unsafe fix and may change runtime behavior
 
-FAST003 [*] Parameter `thing_id` appears in route path, but not in `read_thing` signature
-  --> FAST003.py:24:19
-   |
-24 | @app.get("/things/{thing_id : path}")
-   |                   ^^^^^^^^^^^^^^^^^
-25 | async def read_thing(query: str):
-26 |     return {"query": query}
-   |
-help: Add `thing_id` to function signature
-22 | 
-23 | 
-24 | @app.get("/things/{thing_id : path}")
-   - async def read_thing(query: str):
-25 + async def read_thing(query: str, thing_id):
-26 |     return {"query": query}
-27 | 
-28 | 
-note: This is an unsafe fix and may change runtime behavior
-
 FAST003 [*] Parameter `title` appears in route path, but not in `read_thing` signature
   --> FAST003.py:29:27
    |