fix(relay): add #[allow(dead_code)] to unused items in oauth_token.rs
- Add individual field-level #[allow(dead_code)] annotations to TokenRequestForm fields: code, redirect_uri, client_id, code_verifier, refresh_token (grant_type is read and not annotated)
- Add struct-level #[allow(dead_code)] to TokenResponse (used by future grant implementations)
- Add method-level #[allow(dead_code)] to OAuthTokenError::with_nonce (used by future DPoP nonce error handling)
Follows project convention of field/method-level annotations as seen in app.rs and auth/mod.rs.