Add todo tool

[JonasHelming]

What it does

Adds a TodoList feature to the AI chat system, allowing agents to track task progress during conversations.
The todo tool is currently not integrated in any agent, yet.

• Introduces TodoList interface and TodoListImpl in @theia/ai-chat
• Follows the per-request pattern (like ChangeSet) with ChatTreeTodoList aggregator for branch-aware state
• Adds todoWrite and todoRead tools in @theia/ai-ide
• Persists todo state with chat sessions

How to test

1. Add ~{todoWrite} and ~{todoRead} to an agent prompt and start a chat session
2. Ask the agent to perform a multi-step task and use the todo list
3. Verify the agent can create and update todos via todoWrite
4. Verify the agent can read current state via todoRead
5. Test branching: edit an earlier message, verify the new branch has independent todo state
6. Test persistence: reload the session, verify todos are restored

Follow-ups

Integrate in agents, specifically the Theia Coder agent mode

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
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

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

[sdirix]

I will review at the latest next week.