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](https://spec.commonmark.org/0.31.2/#fenced-code-blocks), 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):

| Opening fence | Closing fence | App Plan renderer | VS Code |
| --- | --- | --- | --- |
| bare ` ``` ` | bare ` ``` ` | Minor glitch: a stray ` ``` ` line leaks out, but content after the block recovers | ✅ correct |
| ` ```csharp ` | bare ` ``` ` | **Broken: bare closer not matched → the block never closes and the entire rest of the document renders as code** | ✅ correct |
| 4-space indented block (no fences) | n/a | ✅ correct | ✅ correct |

### 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.

首先，在桌面应用的 Plan 面板中使用一个包含带语言标签的开放围栏、一个裸结束围栏以及其后的 Markdown 内容的 Markdown 文件复现该问题。当裸结束围栏能够结束围栏代码块，并且后续的标题、引用块、列表和行内代码都能正常渲染，且与 CommonMark 和 VS Code 一致时，修复就完成了。

由索引模型根据 Issue 内容生成。