Fix failed rollouts counted as completed in AgentModeDaemon

[Mr-Neutr0n]

Summary

Fixes #355

• When vLLM returns HTTP 400 errors, rollouts end up with None reward and empty triplets. These were previously counted as "completed" in _async_run_until_finished, causing the completed count to exceed _total_tasks_queued (e.g. "Completed 33/32 tasks") and triggering assertion failures in get_train_data_batch.
• Added _is_failed_rollout() helper that detects rollouts with no reward and no triplets, identifying them as backend failures rather than real results.
• Failed rollouts are now skipped instead of stored, allowing tasks to be re-claimed and retried by the store's timeout mechanism.
• For v1 mode, wait_for_rollouts now only polls for rollout IDs that have not yet successfully completed, so successful retries are properly picked up.
• Removed the early "already processed" skip that could prevent a successful retry from overwriting a stale failed entry.

Test plan

• Deploy with a vLLM backend that intermittently returns HTTP 400 errors and verify that failed rollouts are retried rather than counted as completed
• Verify get_train_data_batch no longer hits the assert len(self._completed_rollouts_v0) == self._total_tasks_queued assertion
• Confirm that "Completed N/M tasks" log messages never show N > M
• Run existing training loop end-to-end to verify no regression in normal (no-error) flow

[Mr-Neutr0n]

Friendly bump! Let me know if there's anything I should update or improve to help move this forward.