[M4n5ter]

Summary

The client cancellation tests deliberately close a query socket with unread data. Their proxy could receive ConnectionReset after every anchor acknowledgement assertion had passed, then fail the test while joining its relay task.

Accept ConnectionReset and BrokenPipe at that query relay boundary. Unexpected I/O errors and worker panics still fail. Both tests continue to require that close waits for the original anchor acknowledgement and that missing acknowledgement returns an error. The change touches one test file.

Verification

• All 28 client library tests pass with compiler warnings denied, including both withheld-opening/anchor-ack cases.
• Client Clippy with all targets and warnings denied passes.
• Rust formatting and diff whitespace checks pass.
• Read-only advisor review found no blocker.
• The parent revision's full CI passed; the companion normal run exposed this proxy teardown race.

AI use

• Generative tooling made a substantive contribution

Tool(s) and scope: OpenAI Codex diagnosed the CI failure, updated the test proxy and ran validation.

Checklist

• Existing cancellation and anchor-custody tests cover the behavior.
• Relevant local tests, lint and formatting pass.

Does this PR entail a change in behavior?

• No