Summary

Adds server-driven retry timing to the HTTP retry pipeline so callers automatically honor Retry-After headers on 429/503 responses, instead of running their own (typically much shorter) exponential backoff inside the server rate-limit window.

• lib-retry: new generic builder hook Retryable.retryDelayHint(Function<R, Duration>). When set, the policy waits min(maxDelay, max(scheduled_backoff, hint)). The hint is a strict lower bound, the configured maxDelay is a strict upper bound, and Failsafe withJitter continues to apply on top. A null hint falls back to the regular exponential backoff — fully backward-compatible for callers that do not opt in.
• lib-httpx: both sendWithRetry and sendWithRetryAsync now wire HxClient::parseRetryAfter as the hint extractor. Only the RFC 9110 §10.2.3 delta-seconds form is parsed (e.g. Retry-After: 41); HTTP-date form degrades to backoff (not used by Seqera services).

Motivation

Nextflow issue nextflow-io/nextflow#7176: the plugin registry returns 429 + Retry-After: 41 under burst CI load, but the HxClient generic exponential backoff (5 attempts ~5.6s total wait) lands every retry inside the same 60-second rate-limit window and they all fail. Honoring Retry-After at the library layer fixes this for every Seqera HTTP caller — Wave, Tower, registry — not just plugin metadata.

Design notes

• withDelayFn and withBackoff are mutually exclusive in Failsafe 3.3.2, so when a hint extractor is registered we compute exponential backoff manually inside the lambda. maxDelay is also enforced manually because withDelayFn does not honor the policy maxDelay field.
• Failsafe rethrows exceptions from delayFn, so the hint extractor invocation is wrapped in try/catch — a misbehaving extractor degrades to backoff rather than breaking the retry.
• Async parity verified with a wiremock test that uses sendAsync(...).get().

Test plan

• :lib-retry:test — 3 new Spock tests for retryDelayHint: floor (hint > backoff), fallback (null hint), cap (hint > maxDelay)
• :lib-httpx:test — 2 new wiremock integration tests (sync and async 429 + Retry-After honored as lower bound), 1 new wiremock test for the maxDelay cap, and 9-row data-driven unit test for parseRetryAfter edge cases (null response, whitespace, zero, negative, non-numeric, fractional, HTTP-date, missing header)
• No regressions in the existing HxClientRetryIntegrationTest, HxClientTest, and RetryableTest suites
• The pre-existing should retry on 429 rate limit integration test pins a small maxDelay so it stays fast under the new behavior

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