fix(design): honor Retry-After header in variants 429 handler

[stedfn]

Closes #1244.

Problem

design/src/variants.ts's 429 handler in generateVariant discards the Retry-After response header and falls through to a local exponential schedule (2s/4s/8s). On image-generation batches, that can burn all retry attempts inside the provider's cooldown window and the request never recovers.

Fix

Parse Retry-After per RFC 7231 — both delta-seconds and HTTP-date. Honored waits capped at 60s to bound stalls from hostile or buggy headers. Delta-seconds validated as digits-only (rejects 2abc). When honored (including 0 and past dates — "retry now"), skip the next iteration's leading exponential sleep so we don't double-wait. Invalid or missing headers fall through to the existing exponential schedule unchanged.

Behavior matrix

Header	Behavior
Retry-After: 5	wait 5s, skip leading on next attempt
Retry-After: 999999	capped to 60s, skip leading
Retry-After: 2abc	invalid, fall through to exponential
Retry-After: 0	wait 0, skip leading (retry immediately)
Retry-After: <past HTTP-date>	wait 0, skip leading
Retry-After: <future date>	wait diff capped at 60s, skip leading
no header	fall through to existing exponential

Test plan

• New: design/test/variants-retry-after.test.ts — 5 cases covering delta-seconds, HTTP-date, invalid, no-header, and Retry-After: 0. Each asserts both the call count and the 1st-to-2nd call timing gap so the fall-through and the no-double-wait properties are both verified.
• generateVariant now accepts an optional fetchFn parameter (default globalThis.fetch) so the tests can inject a stub. Production call sites unchanged.
• Run: bun test design/test/variants-retry-after.test.ts — all 5 pass in ~8s locally.

Notes

• Cap value (60s) is exercised by code review (Math.min(..., MAX_RETRY_AFTER_MS)); not asserted at runtime to keep the test under 10s.
• HTTP-date test uses a 3s offset to avoid second-resolution flake (HTTP dates have second resolution; Date.now() is ms).

[garrytan]

Thanks @stedfn — your fix shipped in v1.30.0.0 (#1391) with credit in the CHANGELOG. Closing since it's already on main. Appreciate the contribution.