fix(sqs): do not treat FIFO empty receives as drained under deferred ack

[Divyanshu Tiwari (divyanshu-tiwari)]

Summary

• Deferred ack leaves SQS FIFO message groups blocked until receipts are deleted, so the next ReceiveMessage can return nothing while the queue still holds work.
• exit_on_empty now skips that empty poll on .fifo queues while this task still holds outstanding receipts; after deletes, the next poll either returns the next batch or a true empty.
• Unbounded join on a FIFO source keeps polling instead of reporting success with leftovers; set a join flush limit as before.

Test plan

• FIFO queue, one message group, more than max_messages bodies, exit_on_empty: true, downstream that completes (not an unbounded join): run drains the queue instead of stopping after the first batch
• Standard queue exit_on_empty still stops on the first empty receive
• Unbounded join on FIFO no longer exits successfully with messages left behind (run keeps polling until a flush limit is set or the process is stopped)