Guard error handler after headers are sent

[charlesedeh021-cell]

Summary

• Adds a
  es.headersSent guard to the global backend error handler.
• Delegates to
  ext(err) when a streamed or partial response has already written headers.
• Prevents the error middleware from attempting a second
  es.status(...).json(...) write after headers are sent.
• Extends the existing error middleware test coverage for the streamed-response case.
• Asserts that
  ext(err) is called and neither
  es.status nor
  es.json is invoked when headersSent is true.

Closes #819
Closes #817
Closes #818
Closes #820

Notes

This keeps the existing error response shapes unchanged. The only behavioral change is for post-header failures, where Express should continue through the normal error finalization path instead of the app trying to write another JSON response.

Testing

• Not run: backend dependencies are not installed in this worktree.
• Ran: git diff --check.