Parse a Retry-After header into a delay in milliseconds, supporting both
the delta-seconds form (Retry-After: 120) and the HTTP-date form
(Retry-After: Wed, 21 Oct 2026 07:28:00 GMT). Returns undefined when the
header is absent or unparseable so the caller can fall back to its own backoff.
Parse a
Retry-Afterheader into a delay in milliseconds, supporting both the delta-seconds form (Retry-After: 120) and the HTTP-date form (Retry-After: Wed, 21 Oct 2026 07:28:00 GMT). Returnsundefinedwhen the header is absent or unparseable so the caller can fall back to its own backoff.