fix: replace old-style <br> tags to fix Mermaid rendering issues

[haocheng6]

Summary

Fixes #13481 to ensure Mermaid renders correctly when the code contains <br> tags. The Mermaid Live Editor also does the same thing to ensure the exported SVG file is valid (see here).

Tip

Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details.

Screenshots

Before	After	Mermaid Code
		See Code 1 below
		See Code 2 below

Code 1

graph TD
    A[Start<br>Initialization] --> B{Condition?};
    B -- Yes --> C[Process A<br>Update Value];
    B -- No --> D[Process B<br>Skip Step];
    C --> E[End];
    D --> E;

Code 2

flowchart TD
    A[Christmas] -->|Get money| B(Go shopping)
    B --> C{Let me
 think}
    C -->|One| D[Laptop]
    C -->|Two| E[iPhone]
    C -->|Three| F[fa:fa-car Car]

Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods