fix: honor HTTP-date Retry-After headers

[Kayvan-Zahiri]

Replicate's retry transport parses date-valued Retry-After headers with datetime.fromisoformat(). Valid HTTP-date values such as Sun, 18 Oct 2026 12:00:30 GMT fail parsing, so a response requesting a 30-second delay is retried after the default 0.1-second backoff.

Add HTTP-date parsing using the standard library, treating timezone-free obsolete HTTP-date forms as UTC. Preserve existing ISO date compatibility, numeric delays, date delay cap, and fallback behavior for expired/malformed values. Both synchronous and asynchronous requests use the same helper.

Validation:

• 44 new controlled HTTPX sync/async retry cases cover 429/503, all three HTTP-date forms, capped future dates, expired dates, numeric values, ISO compatibility, and malformed/empty values. Before: 16 fail, 28 pass. After: all pass.
• Full repository test script with --record-mode=none: 202 passed, 6 skipped; no live predictions or API requests.
• Repository lint script passes: Pyright 0 errors, Ruff lint and formatting pass. Pylint is advisory (--exit-zero) and reports existing unrelated warnings; no warnings on the added parsing code.
• Wheel/sdist build and diff check pass.

HTTP-date contract: https://www.rfc-editor.org/rfc/rfc9110.html#name-retry-after