Improve user experience around slash commands in instant comments

What does this MR do?

Improves user experience and fixes a minor glitches around instant commenting feature introduced with %9.2

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This MR is ~enhancement around Instant comments feature that improves user experience while using slash commands in instant comments.

Screenshots

TBA

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] 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 #32464 (closed)

#27614 (closed)