Add and extend test coverage for models and their methods

Enhanced test coverage includes `repr` methods, model functionality, and validation logic for key models like `GiftItem`, `GiftCategory`, `EventMedia`, `RSVP`, and `GiftPurchase`. Refactored and added fixtures to support comprehensive testing scenarios. Addresses validation for gift reordering and updates `EventMedia` representation format for consistency.

backend/app/models/event_media.py

@@ -46,7 +46,8 @@ class EventMedia(Base, UUIDMixin, TimestampMixin):
     uploader = relationship("User", foreign_keys=[uploaded_by])
 
     def __repr__(self):
-        return f"<EventMedia {self.original_filename} ({self.media_type}) for event={self.event_id}>"
+        return f"<EventMedia {self.original_filename} ({self.media_type.value}) for event={self.event_id}>"
+
 
     @validates('display_order')
     def validate_display_order(self, key, value):

backend/app/models/gift.py

@@ -174,6 +174,13 @@ class GiftCategory(Base, UUIDMixin, TimestampMixin):
         Args:
             gift_ids: List of gift IDs in desired order
         """
+        # Validate gift IDs
+        existing_gift_ids = {gift.id for gift in self.gifts}
+        for gift_id in gift_ids:
+            if gift_id not in existing_gift_ids:
+                raise ValueError(f"Gift ID {gift_id} not found in category")
+
+        # Reorder logic
         gift_order = {gift_id: idx for idx, gift_id in enumerate(gift_ids)}
         for gift in self.gifts:
             if gift.id in gift_order: