#3126 Add deterministic HTTP stress tests

Summary

• exercise HTTP implementations through real TCP connections under concurrent keep-alive, pipelining, fragmented body, connection churn, and incomplete request workloads
• use deterministic seeds so failures can be reproduced locally
• run a bounded smoke test on pull requests and shard 100 seeds across four nightly jobs

Test plan

• scripts/test
• uv run pytest tests/test_stress.py -m stress -n 0
• zizmor .github/workflows/stress.yml

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.