Add comprehensive tests for scheduled task repository, scheduler service, and task handlers

- Implemented tests for ScheduledTaskRepository covering task creation, retrieval, filtering, and status updates.
- Developed tests for SchedulerService including task creation, cancellation, user task retrieval, and maintenance jobs.
- Created tests for TaskHandlerRegistry to validate task execution for various types, including credit recharge and sound playback.
- Ensured proper error handling and edge cases in task execution scenarios.
- Added fixtures and mocks to facilitate isolated testing of services and repositories.

tests/conftest.py

@@ -25,6 +25,7 @@ from app.models.favorite import Favorite  # noqa: F401
 from app.models.plan import Plan
 from app.models.playlist import Playlist  # noqa: F401
 from app.models.playlist_sound import PlaylistSound  # noqa: F401
+from app.models.scheduled_task import ScheduledTask  # noqa: F401
 from app.models.sound import Sound  # noqa: F401
 from app.models.sound_played import SoundPlayed  # noqa: F401
 from app.models.user import User
@@ -346,3 +347,29 @@ async def admin_cookies(admin_user: User) -> dict[str, str]:
     access_token = JWTUtils.create_access_token(token_data)
 
     return {"access_token": access_token}
+
+
+@pytest.fixture
+def test_user_id(test_user: User):
+    """Get test user ID.""" 
+    return test_user.id
+
+
+@pytest.fixture  
+def test_sound_id():
+    """Create a test sound ID."""
+    import uuid
+    return uuid.uuid4()
+
+
+@pytest.fixture
+def test_playlist_id():
+    """Create a test playlist ID.""" 
+    import uuid
+    return uuid.uuid4()
+
+
+@pytest.fixture
+def db_session(test_session: AsyncSession) -> AsyncSession:
+    """Alias for test_session to match test expectations."""
+    return test_session