[Http] Support standard Retry-After header in http_backoff (#4460)
* [Http] Support standard Retry-After header in http_backoff
parse_ratelimit_headers still takes precedence when both are set.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
* much better
* even better
* fix http mocked test
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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>