test: stabilize macOS digest replay server

[ryanfowler]

Summary

• force accepted partial-replay sockets into blocking mode
• reduce the partial response prefix to avoid macOS write stalls

Validation

• cargo fmt --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins
• cargo test --locked --all-features --test http -- --test-threads=2