Honor return annotations of async pytest fixtures

Async fixtures skipped the annotation branch, so a `-> T` annotation was
ignored and only the body was inferred. Infer the annotation directly:
the awaited value for coroutines, and the yielded value for annotated
async generators. Add cases where the body is not inferable.

Also drop the tooling note from the AUTHORS entry.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

AUTHORS.txt

@@ -71,7 +71,7 @@ Code Contributors
 - Dhanraj E (@Dhanu-dynamic) <dhanraj@labs.digiscrypt.com>
 - Charan Rathore (@charan-rathore)
 
-- Abdullah Ahmed (@abdull-ah-med) (with OpenAI Codex assistance)
+- Abdullah Ahmed (@abdull-ah-med)
 
 And a few more "anonymous" contributors.