test(worker): drain sandbox timeout fixtures

[outof-place]

Follow-up to #353 / AWP-125.

Main CI run 33167983332 exposed a test-isolation race: timeout fixtures could leave an abandoned async sandbox collector alive, which later consumed the next test's one-shot runCommand mock and produced undefined.stdout.

This test-only patch:

• waits until the intended mocked SDK call has started before advancing fake timers;
• rejects and drains deferred timeout fixtures after assertions;
• uses persistent stdout mock implementations so a delayed call cannot consume the current test's response;
• keeps the mocked Sandbox import explicit.

Verification:

• poll-agent.test.ts: 21/21, repeated 10/10 sequential runs
• sandbox test directory: 465/465
• worker TypeScript: pass
• git diff --check: pass
• fresh SolAdvisor review: SHIP

No production runtime files are changed.

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