fix: return 0 from delete() when user cancels operation (#1155)

When a user answers "no" to "Commit deletes?", the transaction is
rolled back but delete() still returned the count of rows that would
have been deleted. This was unintuitive - if nothing was deleted,
the return value should be 0.

Now delete() returns 0 when:
- User cancels at the prompt
- Nothing to delete (already worked correctly)

Fixes #1155

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko

src/datajoint/table.py

@@ -977,6 +977,7 @@ def cascade(table):
                     self.connection.cancel_transaction()
                 if prompt:
                     logger.warning("Delete cancelled")
+                delete_count = 0  # Reset count when delete is cancelled
         return delete_count
 
     def drop_quick(self):