fix(proxy/ProviderClient): honour Retry-After on 429 in callProviderWithRetry

[Chen17-sq]

Closes #5672.

Summary

valhalla/jawn's callProviderWithRetry retried 429 responses with the same async-retry exponentialDelay it uses for 5xx, never consulting the provider's Retry-After header. With the defaults that means three retries fire well within most providers' cooldown windows, each one earning another 429 — exactly the loop the original report calls out.

This PR keeps the rest of the retry machinery untouched and adds two pieces:

parseRetryAfter

A small pure helper that accepts either RFC 7231 form:

• delta-seconds — e.g. Retry-After: 30 → 30_000 ms.
• HTTP-date — e.g. Retry-After: Wed, 21 Oct 2026 07:28:00 GMT → milliseconds remaining from now.

It returns null for missing, malformed, or already-in-the-past values so the caller can transparently fall back to the existing backoff. The result is also clamped to 60 s so a hostile or misconfigured upstream cannot freeze the proxy for an unbounded amount of time.

Retry loop change

Inside the async-retry callback, when a 429 is received the proxy now sleeps for the parsed Retry-After value (when present) before throwing. async-retry's exponential backoff still applies on top, so the provider-mandated minimum can only ever lengthen the wait, never shorten one of the existing retries.

if (res.status === 429) {
  const retryAfterMs = parseRetryAfter(res.headers.get("retry-after"));
  if (retryAfterMs !== null && retryAfterMs > 0) {
    await sleep(retryAfterMs);
  }
}
throw new Error(`Status code ${res.status}`);

5xx behaviour is unchanged: 5xx responses still throw immediately and rely on the exponential backoff alone.

Scope

• One file modified: valhalla/jawn/src/lib/proxy/ProviderClient.ts (+50 lines).
• One new unit-test file: valhalla/jawn/src/lib/proxy/__tests__/ProviderClient.test.ts covering the helper end-to-end (delta-seconds, HTTP-date, negative cases, the 60 s cap).
• worker/src/lib/clients/ProviderClient.ts has the same retry pattern and would benefit from the same treatment in a follow-up. I intentionally kept this PR narrow to the file the issue cites; happy to spin off the worker patch in a second PR if you'd prefer that shape.

Tested

$ cd valhalla/jawn
$ npx jest src/lib/proxy/__tests__/ProviderClient.test.ts
PASS src/lib/proxy/__tests__/ProviderClient.test.ts
  parseRetryAfter
    ✓ returns null for missing or empty values
    ✓ parses delta-seconds form into milliseconds
    ✓ trims surrounding whitespace before parsing seconds
    ✓ caps very large delta-seconds at 60 seconds
    ✓ rejects non-numeric, non-date strings
    ✓ parses HTTP-date form into milliseconds from `now`
    ✓ returns null for HTTP-date values in the past
    ✓ caps HTTP-date values far in the future at 60 seconds

Tests: 8 passed, 8 total

$ npx tsc --noEmit       # 0 errors

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