Add branching conversations through an edit message option

[Yahiewi]

This PR closes issue #288 by adding an option to edit previously sent user messages which creates a branch or multiple branches if that message was edited more than once.
This uses updateMessage from @jupyter/chat.

Code changes:

src/components/edit-message-button.tsx and src/index.ts:

• This new React component is added as a section using createEditMessageSection: MessageFooterSection in a new messageFooterRegistry (from @jupyter/chat). It has a BranchNavigation component which shows the branch navigation arrows on edited user messages. It also shows a counter for the branches (for example 2/3). It also preserves scroll position so when editing a message, the conversation isn't scrolled to end.

src/agent.ts:

• Added history getters and setters as well as a truncateHistory(n) function (which keeps the first n turns (user message and response) in the history) in the AgentManager class.
  EDIT: Since a setHistory function was also defined in PR Save and restore chat #303, I renamed the setHistory function defined here to restoreHistory. Although I think it makes more sense to switch the names since the function defined in this PR is a simple setter.

src/chat-model.ts:

• Added _branchPoints private field in AIChatModel class of type Map<string,{ branches: AIChatModel.IBranchSnapshot[]; currentIndex: number } which maps to every edited message messageId an array of branches and the currently active branch index.
• clearMessages method now also clears the branch points.
• Added several functions: editMessage, switchBranch, updateMessage (which is called by @jupyter/chat's native edit UI and basically calls editMessage), getBranchInfo, a new private _snapshotFrom method and refactored out a _createErrorMessage method that's used in editMessage and sendMessage.
• In the AIChatModel namespace, added an IBranchSnapshot interface.

Demo:

Screencast.from.2026-04-07.14-27-32.mp4

[Yahiewi]

I'm thinking about how we can undo the code changes when editing a message, or maybe give the user the option to undo the the code changes. However, I'm still not quite sure how to do this or if this is doable. The main challenge I think is implementing this in a collaborative context:
-If a user undoes his changes by editing the message, should this also undo the changes for his collaborators? What happens if a user is editing a section of the code which is generated by the AI and has been undone by his collaborator? I haven't worked on collaborative issues in Jupyter, so I'm not sure how feasible this is.