WIP: unstick plugin PTY acceptance keep-alive hang on macOS CI

[Hmbown]

Summary

Investigates the required CI / Test (macos-latest) failure on #5407.

Job: https://github.com/Hmbown/CodeWhale/actions/runs/31884025530/job/95018096669

The cucumber test plugin_toml_binary_lifecycle_skill_and_stdio_mcp_acceptance fails at ~82s with no ---- stdout ---- body. The same signature is on origin/main (5ac75add4, job https://github.com/Hmbown/CodeWhale/actions/runs/31870651757/job/94978674482) and the parent. Ubuntu does not run this suite on PRs. This is a pre-existing test-fixture bug, not a #5407 product regression.

82s is ci_scaled(20s) plus cucumber overhead. Slash commands do not need the model; the first chat POST (call plugin echo) is what should surface Do you want to proceed?. The hermetic tiny_http fixture used read_to_string, which waits for EOF. An HTTP/1.1 keep-alive client never closes, so the single-threaded server never answers. GitHub then drops the huge debug_dump panic, which is why the log only names the test.

This change:

• reads Content-Length only and sends Connection: close
• adds a 2s keep-alive POST regression that fails if the fixture hangs again
• CI-scales wait_for_log like every other PTY wait
• fails with a short screen/log dump instead of a 52x200 frame GitHub swallows

Ignoring the required check would be dishonest. Skipping the test would be dishonest. This is a real fixture fix.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked (warning-free under the CI allow list)
• cargo test --workspace --all-features --locked
• Targeted: cargo test -p codewhale-tui --test cucumber --features long-running-tests limited_http_body_stops_at_content_length hermetic_model_server_answers_http11_keepalive_post plugin_toml_binary_lifecycle_skill_and_stdio_mcp_acceptance -- --test-threads=1

Checklist

• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes
• Harvested/co-authored credit uses a GitHub numeric noreply address