fix: stop iterating when batch returns fewer than batch_size candidates

Avoids redundant SELECT queries when the previous batch already
exhausted all matching rows.

Author: chrisburr

diracx-logic/src/diracx/logic/jobs/sandboxes.py

@@ -291,4 +291,8 @@ async def _delete_chunk(chunk: list[int], _sem: asyncio.Semaphore = sem) -> int:
             total_deleted,
         )
 
+        # If we got fewer than batch_size, there are no more candidates
+        if len(sb_ids) < batch_size:
+            break
+
     return total_deleted