Add comprehensive tests for playlist service and refactor socket service tests

- Introduced a new test suite for the PlaylistService covering various functionalities including creation, retrieval, updating, and deletion of playlists.
- Added tests for handling sounds within playlists, ensuring correct behavior when adding/removing sounds and managing current playlists.
- Refactored socket service tests for improved readability by adjusting function signatures.
- Cleaned up unnecessary whitespace in sound normalizer and sound scanner tests for consistency.
- Enhanced audio utility tests to ensure accurate hash and size calculations, including edge cases for nonexistent files.
- Removed redundant blank lines in cookie utility tests for cleaner code.

app/repositories/user.py

@@ -51,6 +51,7 @@ class UserRepository:
 
     async def create(self, user_data: dict[str, Any]) -> User:
         """Create a new user."""
+
         def _raise_plan_not_found() -> None:
             msg = "Default plan not found"
             raise ValueError(msg)