Update editors when ChangeElement added or replaced

[colin-grant-work]

What it does

Fixes #14900 by updating any open editors referring to a given change proposal when the change proposal is updated.

I've put some thoughts together re: this problem here. This PR implements the Monaco editor update solution proposed there. I have a branch that uses the resource-based approach, but that has the problem that we consider the editor 'dirty.' Would be happy to discuss implementing any of the solutions proposed there, or any others. This one isn't my favorite, because it feels like a weird place to assign responsibility for updating the editors, but it does get around the issue of dirty editors most directly.

How to test

1. Open a chat and use a prompt like

@Coder Remove the prefix "MCP" from all user message in packages/ai-mcp/src/browser/mcp-command-contribution.ts

2. The agent should propose a change set. Open a diff for the file affected.
3. Prompt the agent again with something like.

@Coder Actually, let's change the original MCP to CMP instead.

4. The agent should suggest a new set of changes, and the content of the right side of the diff should be updated accordingly.

As I mention over on the issue, this means that any changes the user had made get wiped out. That's probably ok in a case like this, because the user chose to go back to the chat rather than use what had been proposed. On the other hand, if we chose to go with a solution like the Cline extension's, then the file would have been modified in place, and any changes the user made to the suggestions would be picked up by the agent in its second go around. On the other hand, it wouldn't see any MCP's to change to CMP, so it would fail on that count in this case. Lots of tradeoffs when trying to work intelligently with multiple potential versions of the same text 🤷 .

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]

Converting this to a draft for now, because I think I can do better as part of a larger reconfiguration of the AI diff editor UX.