[codex] Improve MarkdownRenderer code blocks

[Xueyang-Song]

Summary

• upgrades MarkdownRenderer code blocks with native chrome, copy actions, line numbers, syntax highlighting support, metadata parsing, diff/highlight states, and improved selection/copy behavior
• adds optional TextMate syntax-highlighting package and sample/docs coverage
• polishes related markdown surfaces including tables, abbreviation tooltips, high-contrast defaults, and accessibility metadata

Details

• introduces code block metadata parsing, rich CodeBlockBox, copy-button localization/accessibility, and provider-based async syntax highlighting
• adds a bounded/provider-aware highlight cache and viewport-biased scheduling
• fixes code-block copy button selection behavior, lazy-reflow handler preservation, code-block chrome drag selection, and heading marker preservation in copied markdown
• adds tests for metadata parsing, TextMate highlighting, source-map heading affixes, cache behavior, high contrast defaults, and keyboard/focus metadata

Validation

• dotnet test .\MarkdownRenderer\MarkdownRenderer.Tests\MarkdownRenderer.Tests.csproj -p:Platform=x64
• dotnet build .\MarkdownRenderer\MarkdownRenderer.SyntaxHighlighting.TextMate\MarkdownRenderer.SyntaxHighlighting.TextMate.csproj -p:Platform=x64
• dotnet build .\MarkdownRenderer\MarkdownRenderer.Sample\MarkdownRenderer.Sample.csproj -p:Platform=x64 -p:OutDir=C:\Users\neroc\.codex\tmp\MarkdownRendererSampleBuild\out\
• dotnet test .\MarkdownRenderer\MarkdownRenderer.PixelTests\MarkdownRenderer.PixelTests.csproj -p:Platform=x64
• dotnet test .\JitHub.WinUI.Tests\JitHub.WinUI.Tests.csproj -p:Platform=x64
• dotnet build .\JitHub.slnx -p:Platform=x64
• dotnet pack .\MarkdownRenderer\MarkdownRenderer\MarkdownRenderer.csproj -p:Platform=x64 -o C:\Users\neroc\.codex\tmp\MarkdownRendererPackages
• dotnet pack .\MarkdownRenderer\MarkdownRenderer.SyntaxHighlighting.TextMate\MarkdownRenderer.SyntaxHighlighting.TextMate.csproj -p:Platform=x64 -o C:\Users\neroc\.codex\tmp\MarkdownRendererPackages

Note: full solution build still reports the existing NETSDK1198 missing win-x64.pubxml warning.