RevocationReason provides structured revocation reasons inspired by RFC 5280. Used in both admin and self-revocation flows. - REVOCATION_REASON_UNSPECIFIED: Default zero value. Use a specific reason; UNSPECIFIED is rejected by admin and self-revocation endpoints. - REVOCATION_REASON_KEY_COMPROMISE: The key was leaked or believed to be in the hands of an unauthorized party. - REVOCATION_REASON_AFFILIATION_CHANGED: The owning actor's relationship with the issuer changed (e.g., role change, departure). - REVOCATION_REASON_SUPERSEDED: A new key has replaced this one as part of a rotation. - REVOCATION_REASON_PRIVILEGE_WITHDRAWN: Admin-only. The actor's privilege to use this key was withdrawn by an operator. Self-revocation requests using this reason are rejected with InvalidArgument. Pair with description on RevokeAPIKeyRequest to record the operator-supplied justification.