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/player.py

@@ -30,10 +30,10 @@ class PlayerStateResponse(BaseModel):
 
     status: str = Field(description="Player status (playing, paused, stopped)")
     current_sound: dict[str, Any] | None = Field(
-        None, description="Current sound information"
+        None, description="Current sound information",
     )
     playlist: dict[str, Any] | None = Field(
-        None, description="Current playlist information"
+        None, description="Current playlist information",
     )
     position: int = Field(description="Current position in milliseconds")
     duration: int | None = Field(