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.

tests/conftest.py

@@ -103,7 +103,8 @@ async def test_client(test_app) -> AsyncGenerator[AsyncClient, None]:
 
 @pytest_asyncio.fixture
 async def authenticated_client(
-    test_app: FastAPI, auth_cookies: dict[str, str],
+    test_app: FastAPI,
+    auth_cookies: dict[str, str],
 ) -> AsyncGenerator[AsyncClient, None]:
     """Create a test HTTP client with authentication cookies."""
     async with AsyncClient(
@@ -116,7 +117,8 @@ async def authenticated_client(
 
 @pytest_asyncio.fixture
 async def authenticated_admin_client(
-    test_app: FastAPI, admin_cookies: dict[str, str],
+    test_app: FastAPI,
+    admin_cookies: dict[str, str],
 ) -> AsyncGenerator[AsyncClient, None]:
     """Create a test HTTP client with admin authentication cookies."""
     async with AsyncClient(
@@ -211,7 +213,8 @@ async def ensure_plans(test_session: AsyncSession) -> tuple[Plan, Plan]:
 
 @pytest_asyncio.fixture
 async def test_user(
-    test_session: AsyncSession, ensure_plans: tuple[Plan, Plan],
+    test_session: AsyncSession,
+    ensure_plans: tuple[Plan, Plan],
 ) -> User:
     """Create a test user."""
     user = User(
@@ -231,7 +234,8 @@ async def test_user(
 
 @pytest_asyncio.fixture
 async def admin_user(
-    test_session: AsyncSession, ensure_plans: tuple[Plan, Plan],
+    test_session: AsyncSession,
+    ensure_plans: tuple[Plan, Plan],
 ) -> User:
     """Create a test admin user."""
     user = User(