Add CancellationTokenSource disposal to CleanUp method

[Copilot]

Addresses feedback from #545 to ensure CancellationTokenSource is disposed in all cleanup paths.

Changes

• Added explicit cts?.Dispose() and cts = null to CleanUp() method in KnowledgeRetrieval.xaml.cs

While CancelResponse() already disposes cts, the explicit disposal in CleanUp() provides defense against future refactoring and makes the cleanup contract clear.

private void CleanUp()
{
    CancelResponse();
    _model?.Dispose();
    _indexer?.RemoveAll();
    _indexer?.Dispose();
    _indexer = null;
    cts?.Dispose();  // Added
    cts = null;       // Added
}

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