fix(types): add future annotations import for S3Client type hints

Add 'from __future__ import annotations' to enable postponed evaluation
of annotations. This allows S3Client type hints (imported only under
TYPE_CHECKING) to work correctly at runtime without NameError.

Fixes unit test collection error:NameError: name 'S3Client' is not defined

src/llama_stack/providers/remote/files/s3/files.py

@@ -4,6 +4,8 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+from __future__ import annotations
+
 import uuid
 from datetime import UTC, datetime
 from typing import TYPE_CHECKING, Annotated, Any, cast