docs: add userinfo 429 backoff guidance

Extend integration guidance to cover provider-side abuse controls on userinfo lookups, advising retry/backoff handling for 429 responses.

docs/INTEGRATION_GUIDE.md

@@ -139,6 +139,7 @@ try {
 - Keep provider URLs in environment variables, not hard-coded.
 - Avoid printing tokens in production pages.
 - Expect `429 Too Many Requests` from provider control endpoints under abuse protection; implement backoff/retry instead of tight loops.
+- Apply the same backoff strategy for provider `userinfo` calls that return `429 Too Many Requests`.
 - If you call provider WebAuthn login challenge/verify endpoints directly, use `POST` only.
 - Configure OAuth client redirect URIs as HTTPS in production; reserve HTTP for localhost loopback testing only.
 - Rotate client secrets for confidential clients.