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/tests/models/test_rsvp.py

@@ -103,4 +103,22 @@ def test_dietary_requirements(rsvp_fixture):
 
     # Update and re-check
     rsvp_fixture.dietary_requirements = "No preference"
-    assert rsvp_fixture.dietary_requirements == "No preference"
+    assert rsvp_fixture.dietary_requirements == "No preference"
+
+def test_rsvp_repr(db_session):
+    # Arrange
+    rsvp = RSVP(
+        id=uuid.uuid4(),
+        event_id=uuid.uuid4(),
+        guest_id=uuid.uuid4(),
+        status=RSVPStatus.ATTENDING,
+        number_of_guests=3,
+    )
+    db_session.add(rsvp)
+    db_session.commit()
+
+    # Act
+    result = repr(rsvp)
+
+    # Assert
+    assert result == f"<RSVP guest_id={rsvp.guest_id} status=attending>"