grant_type is "authorization_code" or "urn:ietf:params:oauth:grant-type:token-exchange".
state correlates authorization-code exchanges with Authorize.
scope is the requested scope for issued tokens.
subject_token is the bearer token being exchanged per RFC 8693.
subject_token_type is the RFC 8693 token type for subject_token.
expires_in is a Gestalt request-side extension for the desired access-token lifetime in seconds. The provider MAY clamp or default it; expires_in in TokenResponse remains authoritative per RFC 6749 §5.1. 0 means use the grant default.
TokenRequest models RFC 6749 token endpoint parameters and RFC 8693 token exchange inputs.