fix: stop retrying when Anthropic bills third-party usage as extra usage

Anthropic now answers some accounts with "Third-party apps now draw from
extra usage, not plan limits". The proxy classified it as an unknown
failure and returned a 500, so OpenCode retried a request that can only
succeed after the user changes their account.

Classify extra-usage and credit errors as billing and return 402, which
OpenCode doesn't retry, with Anthropic's own text.

Tested with a classification and status case in turn-errors-regression.