fix: stabilize compressed SSE retry test

[ryanfowler]

Summary

• keep the compressed SSE test server available across harness retries
• ignore stale empty loopback connections caused by macOS ephemeral port reuse
• shut down the test server cleanly after capturing the retry outcome

Validation

• cargo test --locked --all-features --test http
• repeated targeted test runs
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo fmt --check