markdown: Support triple-tilde Mermaid fences

[KimSeogyu]

Objective

Zed's Markdown parser accepts tilde-fenced code blocks, but Mermaid extraction only strips backtick fences. As a result, a block like this is parsed as Mermaid while the fence itself is still passed to the renderer:

~~~mermaid
graph TD;
~~~

Solution

Teach the Markdown code-block helpers to recognize triple-tilde fences alongside triple-backtick fences.

The change stays in the existing parsing path, so Mermaid rendering does not need a separate special case. A regression test covers extraction from a tilde-fenced Mermaid block.

Testing

• cargo fmt --all -- --check
• cargo test -p markdown (138 tests)
• ./script/clippy -p markdown
• cargo build -p zed
• Opened a ~~~mermaid block in the built Zed Dev app on macOS and verified that Markdown Preview renders the diagram

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

---

Release Notes:

• Fixed Mermaid diagrams in Markdown previews when they use triple-tilde fences.

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: 김서규.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: 김서규.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @KimSeogyu on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[KimSeogyu]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[smitbarmase]

Thanks! I pushed two commits to align the implementation with CommonMark:

• Code fences can contain three or more characters. With a valid block like ~~~~mermaid ... ~~~~, stripping exactly three characters left a trailing ~ in the Mermaid source. We now strip the complete closing fence.
• A closing fence must use the same character as the opener and be at least as long. For example, ```mermaid ... ~~~ is not closed, so we no longer mark it as such.

I added regression tests for both cases.

Thanks again.