test(runner): make the lease-expiry test observe the clock, not race it

[abrichr]

What

Remove the wall-clock race in
TestLeaseDiscipline::test_expired_unrenewed_lease_cannot_report_false_success
(added in #116).

Why

main failed on ubuntu-latest / Python 3.11:

FAILED tests/test_engine/test_runner_loop.py::TestLeaseDiscipline::test_expired_unrenewed_lease_cannot_report_false_success - assert 0 == 1

The test gave the lease an 80 ms life and slept 120 ms inside the run. But
RunnerService._handle_job stages the bundle and binds the effective policy
between reading the lease and the pre-start check
if datetime.now(timezone.utc) >= lease_deadline: raise Refusal(...). When that
work took longer than 80 ms on a loaded runner the dispatch was refused before
execution, so flow.run was never called and len(flow.calls) == 1 failed
with 0.

This is a flaky test, not a product defect: refusing a dispatch whose lease is
already dead is the correct fail-closed behaviour.

How

Drive the sequence by observation instead of by guessed durations:

• Patch engine.runner_loop.datetime with a real clock plus a test-controlled
  offset, and give the lease 30 s so the pre-start check always passes.
• The patched run body waits for the renew loop's first extend attempt, then
  moves the clock 60 s past the deadline, then holds the run open until the
  renew loop stops attempting extends — which proves it observed the expiry.

Every assertion is unchanged: the run executed exactly once, at least one extend
was attempted, the ack outcome is uncertain, no run_summary reached the
wire, and the journal records uncertain. Nothing is skipped or relaxed.

Tests

• -k expired_unrenewed: 5 consecutive runs passed, ~0.16 s each.
• Full suite: 951 passed, 6 skipped.
• ruff check engine/ tests/ scripts/: passed.

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