fix: preserve methods across 301 and 302 redirects

[ryanfowler]

Summary

• rewrite only POST requests to GET for 301 and 302 redirects
• preserve non-POST methods and replayable bodies across redirects
• keep final redirected method and body during Digest authentication retries
• add redirect semantics and Digest integration regression coverage

Validation

• cargo fmt --check
• cargo test --locked --all-features --lib --bins
• cargo test --locked --all-features --test http digest_auth_preserves_put_body_after_found_redirect