Fix SaveXmlInStreamAsync disposing stream prematurely

[Copilot]

SaveXmlInStreamAsync used a using statement that disposed the XmlWriter, closing the underlying ZipOutputStream before all entries were written. Reading back resulted in "root element missing" exceptions.

Changes

• StreamHelper.SaveXmlInStreamAsync: Removed using statement, manually flush without disposal
• StreamHelper.SaveXmlInStream: Added CloseOutput = false for consistency
• Both methods now match behavior: flush writer but leave stream open for caller

// Before: disposed writer, closed stream
using (XmlWriter writer = XmlTextWriter.Create(outStream, settings)) {
    xmlContent.WriteContentTo(writer);
    await writer.FlushAsync();
}

// After: flush only, stream stays open
settings.CloseOutput = false;
XmlWriter writer = XmlTextWriter.Create(outStream, settings);
xmlContent.WriteContentTo(writer);
await writer.FlushAsync();

Async now matches sync pattern - caller owns stream lifecycle.

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