Address five review blockers in the transient link-preview retry path:
- Parse an HTTP-date Retry-After (RFC 9110), not just delta-seconds, so a
dated 429 honors the server cooldown instead of the default window.
- Re-classify the inline retry's own failure so a 429 on attempt two keeps
its Retry-After rather than collapsing to the generic transient window.
- Floor the transient expiry at a positive minimum so Retry-After: 0 cannot
drive a tight refetch loop.
- Schedule each network fetch separately with the backoff sleeping outside
the concurrency scheduler, so a failing URL no longer holds a slot through
its retry window and starves healthy previews.
- Mark permanent validation/policy rejections (SSRF private-host, non-HTTPS
or credentialed URL, bad port, unparseable URL/redirect) so they cache as a
hard miss instead of being inline-retried and self-heal polled every 30s.
Add Rust tests for Retry-After delta-seconds/HTTP-date/past-date/garbage and
the rejection marker, and TS tests for the Retry-After:0 floor, a 429 on the
inline retry, and a permanent rejection skipping retry for the full miss TTL.
Co-authored-by: Taylor Ho <taylorkmho@gmail.com>
Signed-off-by: Taylor Ho <taylorkmho@gmail.com>
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