Add stream-based API and WriteToString() methods to FluentXmlWriter

[Copilot]

Implements two distinct usage patterns: stream-based writing with explicit completion via Done(), and WriteToString() methods as cleaner alternatives to OutputToString().

Changes

Stream-based API

• Start(TextWriter, topLevelElement, FormattingOptions?) - writes directly to provided writer
• Start(Stream, topLevelElement, FormattingOptions?, Encoding?) - creates internal StreamWriter
• Done() - flushes and disposes stream (only disposes when we own the TextWriter)
• Prevents calling OutputToString()/OutputToFile() in stream mode
• Prevents multiple Done() calls with _isDone flag

WriteToString() methods

• WriteToString(), WriteToString(bool), WriteToString(FormattingOptions)
• Delegates to existing OutputToString() implementation

Internal changes

• CustomXmlWriter accepts any TextWriter (not just StringWriter)
• _isStreamMode and _ownsTextWriter flags control behavior and disposal

Examples

Stream-based writing:

using (var fileStream = File.Create("output.xml"))
{
    FluentXmlWriter.Start(fileStream, "root", FormattingOptions.Default.WithSpaces(2))
        .ManySimple(
            SimpleElement.Create("foo").Attr("id", "1"),
            SimpleElement.Create("bar").Attr("id", "2")
        )
        .Done(); // Flushes and disposes
}

String output:

string xml = FluentXmlWriter.Start("root")
    .Complex("child").Text("value").EndElem()
    .WriteToString(indented: true);

All existing API methods remain unchanged and functional. Added 16 tests covering new functionality.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Break down into a streamed version and one that outputs</issue_title>
<issue_description>Have two paths that use the same basic structure.

For the first one, accept a stream in the Start method, and make a Done() method which flushes and disposees the underlying stream:

FluentXmlWriter.Start(File.CreateText(@"/filename.txt"), FormattingOptions.Default, Encoding.UTF8)
    .ManySimple(
        SimpleElement.Create("foo").Attr("id", "1"),
        SimpleElement.Create("bar").Attr("id", "2")
    ).Done(); // Done() flushes and disposes the file stream

For the second one, keep the OutputTo... methods. Instead of a StringBuilder/StringWriter, use a MemoryStream so that you can use the same underlying technology as the first one.

string xml = FluentXmlWriter.Start(File.CreateText(@"/filename.txt"), FormattingOptions.Default, Encoding.UTF8)
    .ManySimple(
        SimpleElement.Create("foo").Attr("id", "1"),
        SimpleElement.Create("bar").Attr("id", "2")
    ).WriteToString(); // Flushes the underlying memory stream and returns the output as a string.
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Break down into a streamed version and one that outputs #14

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