test(e2e): add batch-size cap coverage for SIENTIAPDE-2013

Also fixes the test_activities fixture, which built mongodb_config by
hand and was missing the new required notification_queue_batch_size key
-- every e2e test would have hit a KeyError without this.

New test seeds 5 notifications with a batch size of 2 and confirms three
successive runs return them oldest-first in order (n1,n2 / n3,n4 / n5),
proving the cursor only advances to what was actually returned rather
than skipping documents a capped query didn't fetch. Verified against
real testcontainers Mongo/Redis/Temporal, not mocks.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

e2e/conftest.py

@@ -330,6 +330,7 @@ async def test_activities(
             'connection_string': f'mongodb://localhost:{mongo_port}',
             'database_name': E2E_DATABASE,
             'ttl_index_seconds': 3600,
+            'notification_queue_batch_size': 0,
         },
         email_config={
             'sender_email': 'e2e@example.com',