Fix /assign & /unassign behaviour on Content Editor

What does this MR do and why?

Fixes /assign & /unassign quick action behaviour on content editor to be consistent with plain markdown editor.

References

• !187225 (merged)
• #527159 (closed)

Screenshots or screen recordings

Before	After
Screen_Recording_2025-07-16_at_6.55.41_PM	Screen_Recording_2025-07-16_at_6.52.36_PM

How to set up and validate locally

1. Go to a work item issue
2. Add any assignee
3. Go to comment field and switch to Rich Text Editor
4. Use /assign and /unassign quick actions

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #555137 (closed)