Why: TestIntentSessionLegacyResponsesUnchanged raced the cancel
dispatch against the prior run settling and 409'd under CI load; the
tests now wait for settle and poll busy 409s with a deadline.
Evidence: repair receipt (a1ff-run.json); 3x race runs green
(verify-a1ff.json, exit=0).
Impact: test-only hardening.
Refs: feature/exec-recovery-diagnostics-a2