Rate-limit errors

code: rate_limited is returned with 429 Too Many Requests when an account exceeds its budget.

The only header Permissio sets on rate-limited responses is:

• Retry-After — seconds to wait before the next attempt.

X-RateLimit-Limit and X-RateLimit-Remaining are not emitted today. Do not rely on them; if you need to track headroom, count your own requests.

Implement exponential backoff with jitter on top of Retry-After. Do not loop tightly; many clients run in parallel and the server will not love that.