Kiro returns HTTP 400 with 'Improperly formed request (reset after Xs)'
when a model is not available on that account's subscription tier.
Previously this fell through to COOLDOWN_MS.transient (30s), causing
rapid retries on all accounts before failing — all accounts get locked
simultaneously with no actual fallback.
Treating this as paymentRequired (2min cooldown) ensures:
1. The model is locked on that account for 2min (proper cooldown)
2. The next available account is tried immediately
3. If all accounts hit the same 400, 9Router falls through to the
next provider in the combo
Fixes#384
