fix(server): cancel and await producer task before cleanup to prevent GC warning (Fixes #1121)

[isheng-eqi]

Summary

Fix an asyncio "Task was destroyed but it is pending!" warning in ActiveTask that fires under continuous load when the consumer finishes before the producer on the normal-completion teardown path.

Problem

ActiveTask has two teardown paths with asymmetric producer cleanup:

1. Cancel path (cancel()): cancels _producer_task, agent cancels, waits for _is_finished → clean teardown
2. Normal completion (_run_consumer finally): sets _is_finished, calls _maybe_cleanup() → _on_cleanup(self) → releases ActiveTask reference without cancelling or awaiting the producer

When the consumer finishes first and the reference is dropped, the still-pending producer is GC'd while parked on _request_queue.get() (or inside its finally block at line 566), triggering:

ERROR asyncio Task was destroyed but it is pending!
task: <Task pending name='producer:<uuid>' coro=<ActiveTask._run_producer() running at active_task.py:566>>

Observed 103× over a multi-hour run under continuous load per #1121.

Fix

In _run_consumer's finally block, cancel and await the producer task before _maybe_cleanup(), mirroring what cancel() already does:

if self._producer_task and not self._producer_task.done():
    self._producer_task.cancel()
    with contextlib.suppress(asyncio.CancelledError):
        await self._producer_task

This ensures the producer's finally block (which closes event queues) completes deterministically before the ActiveTask reference is released.

Test Plan

• New test test_producer_cancelled_on_normal_completion: simulates consumer finishing while producer is pending, verifies producer is cancelled + awaited + cleanup callback fires
• ruff check + ruff format + ty check pass
• Existing tests not affected (12 passing, 1 hang is pre-existing Windows asyncio IOCP issue)

Closes #1121

[gemini-code-assist]

Code Review

This pull request ensures that the producer task is cancelled and awaited before cleanup when the consumer finishes first, preventing asyncio "Task was destroyed but it is pending!" warnings. A regression test has been added to verify this behavior. I have no feedback to provide.

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