Stabilize flaky NestedDataLoader test

[glen-84]

Summary

• NestedDataLoader wrapped its work in a hard CancellationTokenSource(2000), so the 2-second budget had to cover the schema build, execution, and snapshot match. Under CI contention that work crossed 2s, the request was canceled, and the test failed intermittently with TaskCanceledException.
• Replaced the fixed deadline with TestContext.Current.CancellationToken, matching the sibling tests in the same file, so the test cancels only on the real test-host token.

Test plan

• Reproduced the original TaskCanceledException by running the full assembly under CPU contention.
• After the change, ran the test 12× under 2× CPU saturation — all pass.

[Copilot]

Pull request overview

This PR stabilizes the flaky NestedDataLoader integration test by removing a hardcoded 2-second CancellationTokenSource timeout and instead using the test host’s cancellation token, aligning the test’s cancellation behavior with other tests in the same file.

Changes:

• Removed CancellationTokenSource(2000) from NestedDataLoader.
• Passed TestContext.Current.CancellationToken into ExecuteRequestAsync and MatchMarkdownAsync to avoid premature cancellations under CI load.

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