Trigger at.js after selecting slash commands with autocomplete suffix

What does this MR do?

• Trigger at.js after inserting (selecting) a slash command that has an autocomplete suffix
• Examples:
  • /labels ~
  • /assign @

I don't think this would be considered a regression. Based on how at.js functions, using the enter for a slash command probably didn't trigger the autocomplete before. I looked into the at.js library, and it resets the focus on the text area when using tab, causing at.js to be retriggered. This is not the case for the enter key, which does not change the focus.

Why was this MR needed?

• This would only work if the slash command was selected using the tab key
• Would not trigger autocomplete after selecting a slash command using the enter key

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27883 (closed)