use aiohttp in remote rollout processor to fix the open files issue

[xzrderek]

Note

Modernizes rollout execution and resource cleanup.

• Remote rollout: migrate from requests to async aiohttp; introduce shared ClientSession with __call__ using async /init POST + timeout and draining; remove /status polling and rely on Fireworks tracing logs; add acleanup() and robust cleanup() that schedules session close when not in an async context
• Base interface: add RolloutProcessor.acleanup() hook
• Call sites: ensure await rollout_processor.acleanup() is invoked in evaluation_test_utils and gepa_trainer finally blocks
• Tests: update to assert acleanup is awaited instead of only cleanup

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