Add validation for display_order and EventMedia tests

Introduce a `validate_display_order` method to ensure a default value of 0 for `display_order`. Extend testing infrastructure with fixtures and unit tests for `EventMedia` to validate properties, metadata, and default behaviors.

backend/tests/conftest.py

@@ -3,6 +3,7 @@ import pytest
 import uuid
 from app.models.user import User
 from app.utils.test_utils import setup_test_db, teardown_test_db
+from app.models import GiftItem, GiftStatus, GiftPriority, RSVP, RSVPStatus, EventMedia, MediaType, MediaPurpose
 
 
 @pytest.fixture(scope="function")
@@ -39,3 +40,74 @@ def mock_user(db_session):
     db_session.add(mock_user)
     db_session.commit()
     return mock_user
+
+
+@pytest.fixture
+def gift_item_fixture(db_session, mock_user):
+    """
+    Fixture to create and return a default GiftItem instance.
+    The event_id, added_by, and other necessary attributes are predefined.
+    """
+    gift_item = GiftItem(
+        id=uuid.uuid4(),
+        event_id=uuid.uuid4(),
+        added_by=mock_user.id,
+        name="Default Gift",
+        description="Default gift description.",
+        price=100.00,
+        currency="USD",
+        quantity_requested=5,
+        quantity_received=0,
+        status=GiftStatus.AVAILABLE,
+        priority=GiftPriority.MEDIUM,
+        is_visible=True
+    )
+    db_session.add(gift_item)
+    db_session.commit()
+    return gift_item
+
+
+@pytest.fixture
+def rsvp_fixture(db_session, mock_user):
+    """
+    Fixture to create and return a default RSVP instance.
+    The event_id and guest_id fields are predefined for testing.
+    """
+    rsvp = RSVP(
+        id=uuid.uuid4(),
+        event_id=uuid.uuid4(),
+        guest_id=uuid.uuid4(),
+        status=RSVPStatus.ATTENDING,
+        number_of_guests=2,
+        response_message="Looking forward to the event!",
+        dietary_requirements="Vegetarian",
+    )
+    db_session.add(rsvp)
+    db_session.commit()
+    return rsvp
+
+
+@pytest.fixture
+def event_media_fixture(db_session, mock_user):
+    """
+    Fixture to create and return a default EventMedia instance.
+    """
+    event_media = EventMedia(
+        id=uuid.uuid4(),
+        event_id=uuid.uuid4(),
+        uploaded_by=mock_user.id,
+        file_path="uploads/sample.jpg",
+        original_filename="sample.jpg",
+        media_type=MediaType.IMAGE,
+        content_type="image/jpeg",
+        file_size=2048,
+        purpose=MediaPurpose.GALLERY,
+        is_public=True,
+        display_order=1,
+        title="Sample Image",
+        description="A sample image for testing.",
+        media_metadata={"resolution": "1920x1080"},
+    )
+    db_session.add(event_media)
+    db_session.commit()
+    return event_media