Plan canvas markdown renderer: language-tagged code fence not closed by a bare closing fence, breaking rest of document

Summary

In the desktop app's Plan canvas (the built-in markdown viewer/editor), a fenced code block that has a language info-string on its opening fence (e.g. ```csharp) is not closed by a bare closing fence (```). The renderer treats the bare closing fence as content and leaves the code block "open," so the entire remainder of the document renders as code — headings (##), blockquotes (>), lists, and inline code all stop being parsed as markdown.

Per the CommonMark spec §4.5, a closing code fence must not carry an info string; a bare ``` is the correct way to close a ```csharp block. VS Code (and GitHub itself) render all the variations below correctly, so the file is valid — the app's renderer is non-conformant.

Repro

Open any .md file in the app's Plan panel containing:

• Opening fence with a language tag (```csharp)
• Some code lines
• A bare closing fence (```)
• Followed by a heading / blockquote / more markdown

Observed behavior by variant (same content, only the fences changed):

Expected

A bare ``` closing fence should close a language-tagged opening fence (```csharp), matching CommonMark and VS Code. Content after the block should resume normal markdown parsing.

Actual

The language-tagged block is only closed by an identically-tagged fence; a spec-correct bare closer is swallowed as content, breaking rendering for everything below the block.

Workaround

• Use an indented (4-space) code block instead of a fenced one, or
• Omit the language info-string so both fences are bare (this still leaks a stray ``` line but at least doesn't break the rest of the doc).

Environment

• GitHub Copilot desktop app, version 1.0.71
• OS: Windows 11
• Surface: Plan canvas (markdown viewer/editor), media=text/markdown

Notes

Screenshots demonstrating all three variants (app vs. VS Code side-by-side) are available and can be attached to this issue.

まず、言語タグ付きの開始フェンス、単独の終了フェンス、その後のMarkdownコンテンツを含むMarkdownファイルを使い、デスクトップアプリのPlanパネルで問題を再現します。単独の終了フェンスでフェンスブロックが終了し、その後の見出し、blockquotes、リスト、インラインコードがCommonMarkおよびVS Codeに一致して通常どおりレンダリングされれば、修正は完了です。