test(buildrunner): [DO NOT REVIEW OR MERGE] prove response-body cancellation

[sbalabanov]

Summary

Intent:

• Provide executable proof for the response-body cancellation discussion on feat(buildrunner extension): add shared platform buildkite/githubactions BuildRunner clients #423.
• This proof-only PR is intentionally closed and must not be reviewed or merged.

Changes:

• Add a synchronized Buildkite client test that waits until response-body reading begins.
• Cancel the request context and assert the blocked read returns context.Canceled.

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Test Plan

Issues

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