fix(backend): check waitlist existence before update in update_waitlist_admin

Added find_unique check before update() call to properly return 404 when
waitlist doesn't exist, following the established pattern used in other
waitlist admin functions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

autogpt_platform/backend/backend/api/features/store/db.py

@@ -2281,6 +2281,17 @@ async def update_waitlist_admin(
     logger.info(f"Updating waitlist {waitlist_id}")
 
     try:
+        # Check if waitlist exists first
+        existing = await prisma.models.WaitlistEntry.prisma().find_unique(
+            where={"id": waitlist_id}
+        )
+
+        if not existing:
+            raise ValueError(f"Waitlist {waitlist_id} not found")
+
+        if existing.isDeleted:
+            raise ValueError(f"Waitlist {waitlist_id} has been deleted")
+
         # Build update data from explicitly provided fields
         # Use model_fields_set to allow clearing fields by setting them to None
         field_mappings = {
@@ -2312,9 +2323,6 @@ async def update_waitlist_admin(
             include={"joinedUsers": True},
         )
 
-        if not waitlist:
-            raise ValueError(f"Waitlist {waitlist_id} not found")
-
         return _waitlist_to_admin_response(waitlist)
     except ValueError:
         raise