Add is_provider_disabled_cheap() so the rotation scan can skip providers
that are disabled — manually via the dashboard toggle or by an auto-disable
cooldown — without constructing the handler. Building a handler validates
credentials, which for some provider types performs a network round-trip, so
a disabled provider was still being contacted. The cheap pre-check reads the
same cache/DB keys as is_rate_limited() and fails safe, leaving the
authoritative check in place.

Also stop recording a failure for non-retryable errors (400/401/403/404/422).
Those are client/configuration problems, not provider-health problems: counting
them tripped the consecutive-failure auto-disable and pulled healthy providers
out of rotation for the whole cooldown.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>