[codex] Add markdown preview content type

[TrueCrimeDev]

Summary

• Add PreviewContentType to Result.PreviewInfo, defaulting to Text for existing plugins.
• Render PreviewContentType.Markdown descriptions in the default preview pane with MdXaml and Flow theme resources.
• Add SDK/JSON-RPC tests for default text behavior, markdown serialization, and Python/JS-style contentType: "markdown" deserialization.
• Include a small CsWin32 compatibility fix for SHParseDisplayName so the current generated bindings compile.

Validation

• dotnet build .\Flow.Launcher\Flow.Launcher.csproj --no-restore
• $env:DOTNET_ROLL_FORWARD='LatestMajor'; dotnet test .\Flow.Launcher.Test\Flow.Launcher.Test.csproj --no-restore --filter "FullyQualifiedName~ResultPreviewTest|FullyQualifiedName~JsonRPCPluginTest.GivenMarkdownPreviewContentType"
• $env:DOTNET_ROLL_FORWARD='LatestMajor'; dotnet test .\Flow.Launcher.Test\Flow.Launcher.Test.csproj --no-build
• dotnet build .\Flow.Launcher.Plugin\Flow.Launcher.Plugin.csproj --no-restore

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Summary by cubic

Adds a markdown preview option for result descriptions and renders it in the preview pane using MdXaml. Hides the old preview chrome when markdown is active, polishes code-block visuals, and fixes emphasis/code rendering; text previews remain the default.

Summary of changes

• Changed
  • Preview pane switches between Text UI and MdXaml based on ContentType; hide icon/title/description chrome when markdown.
  • Disable outer markdown horizontal scrollbar so code blocks manage horizontal scrolling.
  • Normalize MdXaml output to remove stray emphasis markers around code spans and apply bold/italic correctly.
  • Style MdXaml/ICSharpCode.AvalonEdit code blocks at runtime; wrap editors in a rounded frame and retint syntax colors to a softer palette.
  • CsWin32: pass null for optional SHParseDisplayName out param.
• Added
  • PreviewContentType enum (text, markdown) with JSON string serialization.
  • Result.PreviewInfo.ContentType (defaults to Text) and VM flag IsMarkdownPreview.
  • PreviewMarkdownScrollViewer that post-processes the FlowDocument.
  • Theme style PreviewMarkdownStyle (headings, lists, links, code spans/blocks via ICSharpCode.AvalonEdit).
  • Direct AvalonEdit package reference.
• Removed
  • No behavior removed; text previews remain unchanged by default.
• Memory
  • Small, opt-in overhead to render markdown and a one-time FlowDocument traversal. No persistent allocations.
• Security
  • Markdown only; no scripts. Links require user action. No new permissions.
• Unit tests
  • ResultPreviewTest (defaults, lowercase enum serialization) and JsonRPCPluginTest (JSON-RPC contentType: "markdown" deserialization).

Release Note
You can now view rich, formatted markdown in the preview panel, with improved code block styling; regular text previews still work as before.

^{Written for commit e0201cb. Summary will update on new commits. Review in cubic}