fix: cap Retry-After delays

[ryanfowler]

Summary

• cap effective retry delays, including untrusted Retry-After values, at 30 seconds
• warn when a server-provided delay is clamped while respecting --silent
• document the ceiling and add coverage for large delta-seconds and far-future dates

Validation

• cargo fmt
• cargo test --locked --all-features --lib http::retry::tests
• cargo clippy --locked --all-targets --all-features -- -D warnings