Hi everyone, I'd appreciate a PR review for https://github.com/apache/iceberg/pull/16507 when someone has time.
Currently, OAuth2 token endpoint failures (RFC 6749 §5.2) surface as generic `BadRequestException` / `NotAuthorizedException`, with the error type stringified into the exception message. Consumers that triage on the error type must regex-parse the message to recover it. This PR introduces exceptions that carry the OAuth2 error type as a field accessible via a marker interface so that consumers can triage on the exception class reliably. The change is backward compatible. This is a follow-up to #15746 and #16059, applying the same pattern at the OAuth2 token endpoint instead of /v1/config. Thanks in advance, Oguzhan Unlu
