xint-oauth-pkce-state-per-invocation-manual-exchange-procedure

xint generates a new PKCE state and code_challenge on every auth URL invocation. A redirect code from one flow cannot be exchanged in a different invocation (state mismatch error). Save the PKCE verifier before generating the auth URL, then exchange manually via curl: curl -X POST https://api.x.com/2/oauth2/token with verifier, code, client_id, redirect_uri, and grant_type=authorization_code.