Refactor test files for improved readability and consistency

- Removed unnecessary blank lines and adjusted formatting in test files.
- Ensured consistent use of commas in function calls and assertions across various test cases.
- Updated import statements for better organization and clarity.
- Enhanced mock setups in tests for better isolation and reliability.
- Improved assertions to follow a consistent style for better readability.

app/schemas/playlist.py

@@ -1,6 +1,6 @@
 """Playlist schemas."""
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel
 
 from app.models.playlist import Playlist
 from app.models.sound import Sound