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When an Anthropic API key's credit balance is depleted, the upstream
returns HTTP 400 with message containing "credit balance". Previously,
the 400 handler only checked for "organization has been disabled",
so credit-exhausted accounts kept being scheduled — every request
returned the same error.

Treat this case identically to 402 (Payment Required): call
handleAuthError → SetError to stop scheduling the account until
an admin manually recovers it after topping up credits.

Closes #1586