API separates traffic-spike errors from model overload errors

Traffic that ramps too quickly can now return a 429 error with code slow_down, and temporary model overload returns a 503 with code server_is_overloaded. Both may include Retry-After.

Who is affected:

API clients with retry logic keyed on status codes.

What to do:

Update retry logic to honor Retry-After and handle slow_down and server_is_overloaded.