Stabilize StarWars subscription tests against CI timing flake

[glen-84]

Summary

• Three StarWarsCodeFirstTests subscription tests (SubscribeToReview_WithInlineFragment, SubscribeToReview_FragmentDefinition, SubscribeToReview_With_Variables) flaked under the xUnit v3 / Microsoft Testing Platform runner, producing a null snapshot instead of the expected onReview payload (example run).
• They bounded the event wait with a 2000 ms CancellationTokenSource. Under CI thread-pool starvation the event isn't delivered in time, and SubscriptionEnumerator.MoveNextAsync swallows the resulting cancellation, so the read completes with no item and the snapshot mismatches null.
• Switched all three to the pattern already used by SubscribeToReview: acquire the result enumerator before publishing, then MoveNextAsync().AsTask().WaitAsync(TimeSpan.FromSeconds(30)) with Assert.True, so a genuine failure surfaces as a clear TimeoutException rather than a silent null. Also dropped the now-redundant explanatory comment so all four tests are uniform.

Test plan

• dotnet test src/HotChocolate/Core/test/Execution.Tests/HotChocolate.Execution.Tests.csproj --filter-method "*StarWarsCodeFirstTests.SubscribeToReview*" → 12/12 passed across net8.0, net9.0, and net10.0.

[Copilot]

Pull request overview

This PR stabilizes the StarWars code-first subscription integration tests by removing a short, cancellation-based wait that could silently yield no subscription event under CI thread-pool starvation, and replacing it with a longer bounded wait that fails deterministically.

Changes:

• Standardized the three affected subscription tests to acquire the subscription enumerator before publishing the mutation event.
• Replaced the 2s CancellationTokenSource / await foreach pattern with MoveNextAsync().AsTask().WaitAsync(TimeSpan.FromSeconds(30)) plus Assert.True(...) to surface failures as timeouts/assertions instead of null snapshots.
• Removed a now-redundant comment to keep all four SubscribeToReview* tests uniform.

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