Ignore name redef warnings on ImportError

When defining missing optional dependencies from fastapi, mypy issues
errors of the following form: 'Name "FastAPI" already defined
(possibly by an import) [no-redef]'. This commit fixes such mypy errors.

Author: darkhaniop

src/a2a/server/apps/jsonrpc/fastapi_import_helpers.py

@@ -22,11 +22,11 @@ def __init__(self) -> None:
             """Raises ImportError when initiating a dummy fastapi.* instance."""
             raise ImportError(_FASTAPI_DEPENDENCY_ERROR_MSG)
 
-    class FastAPI(_DummyFastAPIClasses):
+    class FastAPI(_DummyFastAPIClasses):  # type: ignore[no-redef]
         """A dummy fastapi.FastAPI declaration."""
 
-    class Request(_DummyFastAPIClasses):
+    class Request(_DummyFastAPIClasses):  # type: ignore[no-redef]
         """A dummy fastapi.Request declaration."""
 
-    class Response(_DummyFastAPIClasses):
+    class Response(_DummyFastAPIClasses):  # type: ignore[no-redef]
         """A dummy fastapi.Response declaration."""