Convert markdown to XML documentation syntax in C# emitter

[Copilot]

C# XML documentation doesn't support markdown syntax. The emitter was generating markdown (**bold**, *italic*, - bullets) in XML doc comments, causing incorrect rendering in IntelliSense and documentation tools.

Changes

DocHelpers.cs

• Added ConvertMarkdownToXml() to transform markdown syntax to XML tags
• Converts inline formatting: **bold** → <b>, *italic* → <i>, ***both*** → <b><i>
• Converts lists: - item → <list type="bullet">, 1. item → <list type="number">

XmlDocStatement.cs

• Extended SkipValidTag() to recognize converted XML tags (<b>, <i>, <list>, <item>, <description>)
• Prevents escaping of generated XML documentation tags

DocHelpersTests.cs

• Added 16 test cases covering all conversion scenarios

Example

Before:

/// <summary>
/// Bullet point with **bold text**.
/// - One: description
/// - Two: description
/// </summary>

After:

/// <summary>
/// Bullet point with <b>bold text</b>.
/// <list type="bullet"><item><description>One: description</description></item><item><description>Two: description</description></item></list>
/// </summary>

Original prompt

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This section details on the original issue you should resolve

<issue_title>Adopt Documentation spector scenarios</issue_title>
<issue_description>https://github.com/microsoft/typespec/tree/main/packages/http-specs/specs/documentation</issue_description>

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

• Fixes Adopt Documentation spector scenarios #9393

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[jsquire]

I wonder if we should consider hoisting all the allowed tags into some HashSet or const class somewhere.

[jsquire]

Should we consider hoisting the RegEx and making it a static pre-compiled?