Merge upstream main at 60b35a1ee0 without rewriting the existing PR commits.
Keep upstream tracing, held-queue behavior, and test cleanup while retaining
the PR's boundary delivery and cancellation coverage.

The user authorized this single conflict-resolution update after maintainer
takeover. Further maintenance remains subject to the takeover restriction.

Co-Authored-By: GPT-6.1 Sol at high effort <noreply@openai.com>
Co-Authored-By: GPT-6.1 Sol at high effort, implementation subagent for merge conflicts <noreply@openai.com>
Co-Authored-By: GPT-6.1 Sol at xhigh effort, independent merge review <noreply@openai.com>