fix(note-editor): MathJax Block

[david-allison]

Note

Assisted-by: Claude Opus 4.7

Purpose / Description

Was: \[\\]
Now: \[\]

Fixes

• Fixes MathJax block button adds an extra backslash: \[\\] #20961

How Has This Been Tested?

Unit tested

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner