refactor: Improve exception handling and logging in authentication and playlist services; enhance code readability and structure

app/services/auth.py

@@ -467,11 +467,13 @@ class AuthService:
         # If user has existing password, verify it
         if had_existing_password:
             if not current_password:
-                raise ValueError("Current password is required when changing existing password")
+                msg = "Current password is required when changing existing password"
+                raise ValueError(msg)
             if not PasswordUtils.verify_password(current_password, user.password_hash):
-                raise ValueError("Current password is incorrect")
+                msg = "Current password is incorrect"
+                raise ValueError(msg)
         else:
-            # User doesn't have a password (OAuth-only user), so we're setting their first password
+            # User doesn't have a password (OAuth-only user), setting first password
             logger.info("Setting first password for OAuth user: %s", user_email)
 
         # Hash new password
@@ -509,6 +511,6 @@ class AuthService:
             updated_at=user.updated_at,
         )
 
-    async def get_user_oauth_providers(self, user: User):
+    async def get_user_oauth_providers(self, user: User) -> list:
         """Get OAuth providers connected to the user."""
         return await self.oauth_repo.get_by_user_id(user.id)