[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 pane with code highlighting and images, plus smarter preview suppression and icon/logo fallbacks for more reliable visuals. Default text previews remain unchanged.

Summary of changes

• Changed
  • Preview pane switches between Text UI and MdXaml; hides old chrome for markdown and lets code blocks own horizontal scrolling.
  • Adjusted preview layout widths; switched search/settings icons to PNG.
  • Results without icons inherit the plugin icon and fall back to a generic glyph if still empty.
  • Preview can be suppressed per-result (e.g., hidden content); restores when selecting a normal result.
  • Image loading logs warnings instead of errors on failures and constrains wide icons to the small-icon box.
  • UWP logos resolve localized/nested assets and use larger tiles for preview; Win32 and .lnk parsing handle COM/missing-path cases without noisy logs.
  • Unobserved task exceptions are logged without destabilizing DEBUG sessions.
• Added
  • PreviewContentType now supports text, markdown, and hidden; Result.PreviewInfo defaults to text.
  • PreviewMarkdownScrollViewer and CodeHighlightTheme to render markdown with AvalonEdit highlighting; normalized emphasis around code spans and rounded code-block frames.
  • Theme style PreviewMarkdownStyle and a direct AvalonEdit package reference.
  • Tests covering preview behavior, JSON-RPC contentType: "markdown" deserialization, image loading/fallbacks, UWP/Win32 logo resolution, and results collection updates.
• Removed
  • No behavior removed; text previews remain default.
• Memory
  • Small, opt‑in overhead for markdown rendering and a one-time document pass; no persistent allocations.
• Security
  • Markdown only; no scripts. Links require user action. No new permissions.
• Unit tests
  • ResultPreviewTest, PreviewMarkdownScrollViewerTest, PreviewMarkdownStyleTest, MainViewModelPreviewTest, ResultsViewModelTest, ResultViewModelPreviewImageTest, ImageLoaderTest, UwpPackageLogoTest, Win32ProgramTest, JsonRPCPluginTest.

Release Note
You can now view rich Markdown (with code highlighting and images) in the preview panel; icons and app logos load more reliably, and text previews still work as before.

^{Written for commit b8cf3fa. Summary will update on new commits.}