Fix issue in RTE related to adding text before a mention

What does this MR do and why?

Fix issue in RTE related to adding text before a mention. This was a regression from !158367 (diffs).

If you start the input by tagging ( @somebody) someone, and then relocate the cursor to before the @, you could type to insert text, but couldn't delete or otherwise interact with it (backspace and left arrow keys don't respond). When switching to plain-text, anything typed before the @ disappeared. This commit fixes the issue.

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Screenshots or screen recordings

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Before	After
Screen Recording 2024-07-17 at 12.48.55.mov	Screen Recording 2024-07-17 at 12.47.58.mov

How to set up and validate locally

1. In an issue comment, switch to rich text editor.
2. Mention a user.
3. Move cursor to before the mention.
4. Type something.
5. Switch to plain text editor.
6. The text before the mention should not disappear.

Related to #472988 (closed)