Wait for the fast rollout before timing gather's partial return

[yichao-liang]

Summary

tests/agent_sdk/test_async_rollouts.py::test_gather_timeout_returns_partial failed in a CI-container replay of shard 7: gather([fast, slow], timeout=2) returned with the trivial fast rollout still pending.

The test raced a forked child's start-up against a 2 s timeout.
Under pytest-cov every forked child starts its own coverage tracer, and in a shard with thousands of modules imported on a busy node that start-up alone can pass 2 s.
Measured on a quicktest node with 5,600 modules imported, launch to result took:

• 0.06 s without coverage,
• 0.3 s with coverage,
• 1.0 to 2.9 s with coverage under CPU contention (the first launch took 2.86 s).

The test now waits for the fast child on its own first, then checks that a short gather timeout returns it as done and the slow child as pending without blocking.
The registry is unchanged.

Independent of the #189-#192 and #194 stack; it touches only this test file.

Test plan

• Reproduced the start-up latency above with coverage on and the CPU contended.
• Under the same conditions, test_async_rollouts.py passes with the fix (all 9 tests, including the overlap test next to it).
• pylint, mypy, yapf and docformatter on the file.

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