test(worker): bind sandbox mock statically

[outof-place]

Follow-up to #354 / AWP-125.

Main CI run 33173321742 showed that the top-level dynamic test import introduced in #354 could still lose the Vitest mock boundary under the full suite: one timeout test never reached its stdout spy and the following replay test reached the real Vercel Sandbox OIDC path.

This surgical test-only patch restores the static Sandbox import while retaining #354's deferred rejection/drain cleanup. Vitest hoists vi.mock, so runtime dynamic imports in poll-agent.ts resolve against the statically established mocked module.

Verification:

• poll-agent.test.ts: 21/21, repeated 20/20 sequentially (420 tests)
• worker TypeScript: pass
• git diff --check: pass
• task-scoped Sol profile exactness check: pass
• fresh Sol/High audit: SHIP

No production runtime file changes.

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Project	Deployment	Actions	Updated (UTC)
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