Fix WhenOnDisposeAsyncSourceFails_ThenCallbackInvoked to actually assert callback invocation

[Copilot]

The test WhenOnDisposeAsyncSourceFails_ThenCallbackInvoked was asserting only on the completion result, not on whether the OnDispose callback itself was ever called — meaning the test would pass even if OnDispose was never invoked.

Change

Added a disposeCallbackInvoked flag set inside the OnDispose lambda, with an explicit assertion that it becomes true:

var disposeCallbackInvoked = false;
var source = ObservableAsync.Throw<int>(new InvalidOperationException("fail"))
    .OnDispose(() =>
    {
        disposeCallbackInvoked = true;
        return default;
    });

// ...

await Assert.That(completionResult!.Value.IsFailure).IsTrue();
await Assert.That(disposeCallbackInvoked).IsTrue(); // ← added

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[github-actions]

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