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.

Start by reproducing the issue in the desktop app's Plan panel with a markdown file containing a language-tagged opening fence, a bare closing fence, and markdown content afterward. The fix is complete when the bare closer ends the fenced block and subsequent headings, blockquotes, lists, and inline code render normally, matching CommonMark and VS Code.

Written by the indexing model from the issue text.