Buffers built for the commit view (git history -> open excerpts) were
assigned a language without attaching the language registry, so injected
grammars (Markdown-Inline, fenced code block languages) could not be
resolved and their captures went unstyled.

Attach the registry before assigning the language, matching the pattern
in text_diff_view.rs, and add a regression test asserting that a buffer
built via build_buffer parses with Markdown-Inline layers.

Fixes #64577 

# Objective

- Describe the objective or issue this PR addresses.
- If you're fixing a specific issue, use "Fixes #X" for each issue as
[described in the GitHub
docs](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword).

## Solution

- Describe the solution used to achieve the objective above.

## Testing

- Did you test these changes? If so, how?
- Are there any parts that need more testing?
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?

## Self-Review Checklist:

- [x ] I've reviewed my own diff for quality, security, and reliability
- [ x] Unsafe blocks (if any) have justifying comments
- [ x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ x] Tests cover the new/changed behavior
- [x ] Performance impact has been considered and is acceptable

## Showcase

> This section is optional. If this PR does not include a visual change
or does not add a new user-facing feature, you can delete this section.

- Help others understand the result of this PR by showcasing your
awesome work!
- If this PR includes a visual change, consider adding a screenshot,
GIF, or video
- A before/after comparison is very useful for changes to existing
features!

While a showcase should aim to be brief and digestible, you can use a
toggleable section to save space on longer showcases:

<details>
  <summary>Click to view showcase</summary>

<img width="1170" height="831" alt="image"
src="https://github.com/user-attachments/assets/56d948b0-b9da-42ec-abb2-3b19a7b38948"
/>

</details>

---

Release Notes:

- Fixed syntax highlighting for Markdown emphasis, links, and code
blocks when viewing a file at a historical commit