fix: replace deprecated .dict() with .model_dump() and fix db.add() AsyncMock warnings

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/ac39d3f7-d4c3-4417-8c33-8e6340a27281

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/api/v1/endpoints/admin.py

@@ -322,7 +322,7 @@ async def create_admin_notification_config(
     db: AsyncSession = Depends(get_db),
 ):
     """Create a new admin notification configuration (admin only)"""
-    config = AdminNotificationConfig(**config_in.dict())
+    config = AdminNotificationConfig(**config_in.model_dump())
     db.add(config)
     await db.commit()
     await db.refresh(config)
@@ -370,7 +370,7 @@ async def update_admin_notification_config(
             detail="Admin notification config not found",
         )
 
-    update_data = config_in.dict(exclude_unset=True)
+    update_data = config_in.model_dump(exclude_unset=True)
     for field, value in update_data.items():
         setattr(config, field, value)