Function parse_retry_after 

pub fn parse_retry_after(
    retry_after: Option<&str>,
    retry_after_ms: Option<&str>,
) -> Option<Duration>

Parse a Retry-After or retry-after-ms header value into a Duration.

Supports:

• retry-after-ms header (milliseconds, if provided separately)
• Retry-After as seconds (integer or float)
• Retry-After as HTTP date (RFC 2822)

Returns None if parsing fails.