fix: persist sessions after response errors

[ryanfowler]

Summary

• save session cookie changes even when redirect, replay, digest, signing, or later request processing fails
• preserve dry-run behavior by skipping session writes
• cover invalid redirects and malformed dry-run sessions with integration tests

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --test http session_cookies_persist_after_invalid_redirect
• cargo test --locked --all-features --test http dry_run_leaves_corrupted_session_unchanged