Give each SSE response its own header list
Merged
KL
18d ago+45+45+01234567890123456789-12-12-01234567890123456789
Closes #3701.
handle_modern_request passed the module-level _SSE_HEADERS list straight into http.response.start. Middleware that appends to message["headers"] in place mutated that shared list, so every later SSE response carried all previously appended headers. Each SSE response now gets its own copy.
AI Disclaimer
This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.