Fix dispose methods to always set _isDisposed and call base.Dispose() when LeaveOpen is true

[Copilot]

The dispose logic in SharpCompressStream returned early when LeaveOpen was true without setting _isDisposed or calling base.Dispose(), allowing subsequent dispose calls to re-execute disposal code.

Changes:

• Restructured Dispose() and DisposeAsync() to always set _isDisposed and call base disposal methods
• LeaveOpen check now only guards wrapped stream disposal and buffer cleanup
• Verified no other dispose methods in codebase have this pattern

Before:

protected override void Dispose(bool disposing)
{
    if (_isDisposed || this.LeaveOpen)
    {
        return;  // Never sets _isDisposed or calls base.Dispose() when LeaveOpen=true
    }
    _isDisposed = true;
    base.Dispose(disposing);
    // ...
}

After:

protected override void Dispose(bool disposing)
{
    if (_isDisposed)
    {
        return;
    }
    _isDisposed = true;
    base.Dispose(disposing);
    if (this.LeaveOpen)
    {
        return;  // Skip only stream disposal and buffer cleanup
    }
    // ...
}

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