fix(batch): prevent race condition in batch queue process loop

[kbak]

Summary

• Fix race condition where the one-shot process_loop could exit after a single flush while new requests were being enqueued, leaving them stranded with no processor
• The spawner would skip re-spawning because task_running was cleared outside the pending lock, creating a window where enqueue() sees task_running=true but the task is about to exit
• process_loop now continuously flushes every max_wait_ms and only exits when the queue is empty, atomically clearing task_running under the pending lock to close the race window

Test plan

• Run existing batch settlement tests
• Stress test with concurrent settle requests to verify no requests are dropped
• Verify batch processor exits cleanly when queue drains

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