Explore a controller based chat session item API

[mjbvz]

For #276243

Explores moving the chat session item api to use a controller instead of a provider. This uses managed objects and is based on the TestController we already have finalized

Main flow :

• Extensions call createChatSessionItemController('my.sessionId') to create the controller. The controller is a managed object with a dispose method

• VS Code then calls refreshHandler on the controller to build up the initial list

• During refresh, the extension updates the ChatSessionItemCollection managed object. This object is basically just a managed array. This lets the editor see changes that are made to the session list

A few other points:

• Chat session items would now also be managed objects. Extensions could set properties to update them in the UI

• If an extension needs to update the list of chat session items independently of VS Code — for example if a new session is created through GitHub.com — it would simply update the ChatSessionItemCollection

• We would have event handlers for when the editor archives or disposes of a chat session item

Still some open questions but seems promising. Also although the general API shape looks pretty different , the logic shouldn't be that different for extensions

[Copilot]

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