Fix race condition in CancellationTokenSource disposal

[Copilot]

The CancelCTS() method in SampleContainer had a race condition where concurrent calls could cause double-disposal of CancellationTokenSource. The pattern of reading, nulling, and disposing without synchronization allowed multiple threads to read the same non-null reference before any thread nulled it.

Changes:

• Added _ctsLock object to serialize all _sampleLoadingCts access
• Wrapped read-and-null operations in CancelCTS() with lock
• Protected CancellationTokenSource creation/disposal in LoadSampleAsync with lock

Before:

private void CancelCTS()
{
    var cts = _sampleLoadingCts;
    if (cts == null) return;
    
    _sampleLoadingCts = null;  // Race: multiple threads can read before this executes
    using (cts) { cts.Cancel(); }
}

After:

private void CancelCTS()
{
    CancellationTokenSource? cts;
    lock (_ctsLock)
    {
        cts = _sampleLoadingCts;
        _sampleLoadingCts = null;  // Atomic with read
    }
    
    if (cts != null)
    {
        using (cts) { cts.Cancel(); }
    }
}

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