Fix SA1214 and IDISP003 analyzer violations in SampleContainer

[Copilot]

Addresses two analyzer errors introduced in commit d8aa456:

• SA1214: Readonly field declared after non-readonly fields
• IDISP003: Field reassigned without disposing previous value

Changes

• Field ordering: Moved _ctsLock readonly field before non-readonly fields to satisfy StyleCop conventions
• Disposal pattern: Refactored CancelCTS() to capture and null the field inside lock, then cancel and dispose outside lock

private void CancelCTS()
{
    CancellationTokenSource? cts;
    lock (_ctsLock)
    {
        cts = _sampleLoadingCts;
        _sampleLoadingCts = null;
    }

    if (cts != null)
    {
        try
        {
            cts.Cancel();
        }
        finally
        {
            cts.Dispose();
        }
    }
}

This avoids holding the lock during potentially long cancel operations while ensuring proper disposal ordering.

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