fix: Improve indexing of the request_queue_records table for SqlRequestQueueClient (#1527)

### Description

- This PR improves the index for the `request_queue_records` table, as
the previous version of the index was not efficient for retrieving
requests.

EXPLAIN result for over a million requests
Old index

<img width="1133" height="573" alt="image"
src="https://github.com/user-attachments/assets/6c75d6ac-bd86-4b8e-9abf-3dc897a42ad7"
/>

New index

<img width="1028" height="443" alt="image"
src="https://github.com/user-attachments/assets/e79780e8-fba7-42e2-aa47-f70d83e8c372"
/>

### Issues

- Closes: #1526

Author: Mantisus

src/crawlee/storage_clients/_sql/_db_models.py

@@ -205,9 +205,8 @@ class RequestDb(Base):
             'idx_fetch_available',
             'request_queue_id',
             'is_handled',
-            'time_blocked_until',
             'sequence_number',
-            postgresql_where=text('is_handled = false'),
+            postgresql_where=text('is_handled is false'),
         ),
     )