Follow-up on the salvage of #59523. Two low-risk cleanups surfaced by review:
- Extract _ZAI_CODING_OVERLOAD_SHORT_ATTEMPTS as a module constant so
adaptive_rate_limit_backoff() and zai_coding_overload_retry_ceiling()
share one source of truth. Previously both hardcoded short_attempts=3
independently; tuning one without the other would silently desync the
retry ceiling from the backoff schedule.
- Replace the tautological formula-mirroring assert in
test_zai_overload_retry_ceiling_exceeds_short_attempts with a behavior
invariant (ceiling leaves headroom for every long-backoff entry), per the
repo's contracts-over-snapshots testing rule.
Assert the invariant that the Z.AI overload retry ceiling exceeds the
short-retry threshold (the original bug had them equal, so the long tier
was dead code), and walk the attempt range the retry loop actually
traverses to prove the full 30/60/90/120s long-backoff schedule now runs.
