WIP: Use langgraph to call tools

[brichet]

This PR adds langgraph, which allows to call tools that can interact with Jupyterlab API.

As an example, the PR includes a default tools, that only shows a Jupyterlab modal with the response from the model.

record-2025-06-26_17.23.44.webm

Current state

• add a setting to enable the use of tool (only a test tool available as of now)
• create an agent (LLM + tool) using langgraph, and call it instead of only the LLM
• the LLM decides by itself if it should call a tool or not. The PR updates the system prompt to require calling it, for now.
• add a message for each message received from the LLM (user is Jupyternaut) and from the tool (user is the tool name)

To do, To discuss

• add a registry (with a token) to enable additional tools from extensions
• @jtpio suggested to switch from LLM to agent from the chat panel itself (instead of the settings). If we do that, we should probably be able to select the tool(s) to use from the chat panel too.
• currently there is an additional instruction in the system prompt to request the use of the tool. We should explore if there is an option when binding the tool to the model, if we want to force the use of it.
• we should probably pass only a subset of the agent for security reason (as we did for chat model and completer model in Do not expose providers api #84)