scripts(evals): run evals in parallel

[aconchillo]

Please describe the changes in your PR. If it is addressing an issue, please reference that as well.

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[JiwaniZakir]

In run_eval, the Daily room is created near the top of the function but deleted only at the bottom — outside any try/finally block. If an exception escapes after room creation (e.g., during task setup or the asyncio.wait_for call), the room will leak. Wrapping the body after room = await helper.create_room(...) in a try/finally that always calls delete_room_by_name would make the cleanup unconditional.

Additionally, two separate aiohttp.ClientSession instances are opened per eval (one for creation, one for deletion). Since these sessions are very short-lived and the client is stateless, this works correctly, but it's worth either reusing a single session within run_eval or sharing a session at the EvalRunner level across all concurrent evals to reduce connection overhead when many evals run in parallel.

The refactor to _create_eval_callbacks returning closures over a per-eval result_future is clean and correctly removes the shared self._result_future state that would have caused race conditions under parallel execution. The result_future.done() guard in assert_eval is a good addition for the case where both EndFrame and CancelFrame could fire close together.