Add batch rollout helpers and multi-task queueing

[ultmaster]

Summary

• allow training_rollout_batch/validation_rollout_batch APIs to receive per-task resource lists
• add batch_size to AgentRunner and propagate through Trainer for batched polling
• test batch rollouts with resource lists

Testing

• pre-commit run --files agentlightning/litagent.py agentlightning/runner.py agentlightning/trainer.py tests/test_litagent.py
• pytest tests/test_litagent.py tests/test_client.py

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https://chatgpt.com/codex/tasks/task_e_689db4c153ec832ea2983ab0885c7f75