test: make lease-expiry integration test deterministic

[nficano]

Summary

LeaseExpiryTests.ValidateOpAsync after expires_at raises LEASE_EXPIRED gave only a 50ms window between submit and lease expiry. On a loaded CI runner, submit-time validation saw the lease as already expired and rejected the submit with INVALID_REQUEST instead of job.accepted (observed on PR #92's test run).

• Widen expires_at to 1s so submit-time validation can't lose the race.
• Replace the fixed 150ms agent delay with a ValidateOpAsync poll loop, so the first post-expiry call raises LEASE_EXPIRED deterministically.

Verified locally: 5 consecutive runs of the lease-expiry tests plus the full 268 unit + 28 integration suite, all green.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between b0f624b and 7af5602.

📒 Files selected for processing (1)

• tests/Arcp.IntegrationTests/LeaseExpiryTests.fs

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch fix/flaky-lease-expiry-test

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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