[Shubham-Padkonde]

Change summary

Update the client's rate-limit counters from valid response headers before returning an HTTP error. Previously, a 429 response left RateLimitRemaining() and RateLimitReset() at their previous values, making client-level backoff decisions use stale information.

The original response and error are still returned. Missing or malformed headers retain the prior values, canceled requests with no response are safe, and GET/HEAD behavior is unchanged.

Fixes #857.

All Submissions:

• Have you followed the guidelines in our Contributing document? Validation details and platform limits are below; the changelog links to this PR.
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes to Core Features

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

Validation after merging current main (Linux, Go 1.26.10):

• make all passes: module download/tidy, golangci-lint formatting and lint, and the full recorded-fixture test suite.
• The changelog entry remains under Unreleased, above the v18.0.0 release.
• git diff --check passes; module files unchanged.
• Semgrep is not installed, so the Makefile's optional Semgrep step did not run. Race detection and live Fastly API tests were not run.
• The original regression validation demonstrated four failing cases before the fix.

User Impact

Callers can read updated rate-limit state through the existing client accessors after rejected write requests.

Release considerations

No public API changes or new dependencies. Prepared and tested with Codex assistance.

[anthony-gomez-fastly]

LGTM, thank you for your contribution!

[anthony-gomez-fastly]

@Shubham-Padkonde please fix the changelog and it should be good to go once @rcaril reviews as well