15048: delete the change set when all changes are deleted

[CamilleLetavernier]

fixes #15048

What it does

• Delete the entire ChangeSet from the Chat when the last change is deleted

How to test

• Ask Coder to provide changes (on one or several files), for example:

Can you suggest 3 arbitrary changes to improve the code in #currentRelativeDirPath ? This is just a test, so be creative. Changes should affect multiple files

• Delete all changes one by one
• Once the last change is deleted, the change set should disappear as well (instead of remaining empty)

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[colin-grant-work]

Behavior seems reasonable to me. Possibly we should also make allowances in the UI for empty change sets, though, rather than relying on their deletion?