Add comprehensive tests for player API endpoints and player service functionality

- Implemented tests for player API endpoints including play, pause, stop, next, previous, seek, set volume, set mode, reload playlist, and get state.
- Added mock player service for testing API interactions.
- Created tests for player service methods including play, pause, stop playback, next, previous, seek, set volume, set mode, and reload playlist.
- Ensured proper handling of success, error, and edge cases in both API and service tests.
- Verified state management and serialization in player state tests.

app/core/database.py

@@ -38,6 +38,13 @@ async def get_db() -> AsyncGenerator[AsyncSession, None]:
             await session.close()
 
 
+def get_session_factory():
+    """Get a session factory function for services."""
+    def session_factory():
+        return AsyncSession(engine)
+    return session_factory
+
+
 async def init_db() -> None:
     """Initialize the database and create tables if they do not exist."""
     logger = get_logger(__name__)