fix(malformed): Retry-After header parsing

[farquadars]

Summary

Fix Retry-After parsing so malformed numeric-like values are no longer treated as valid retry delays.

What changed

• only accept fully numeric integer Retry-After values
• reject malformed values such as 5s and 1.5
• add unit tests for these edge cases

This prevents incorrect retry delays when a proxy or upstream service returns an invalid Retry-After header.

[ryanio]

Solid fix — the current parseInt silently accepts malformed values like "5s". The tighter regex and Number.isSafeInteger check are the right approach. Tests look good too. LGTM!