Debt- clean up async ChatModel/IChatEditingSession initialization

[roblourens]

The ChatModel and editing session intialization got complicated after unifying the chat views and loading sessions from disk, async, instead of the storage service, sync.

Since the editing session setup blocks on the ChatModel being ready in the ChatService, the ChatModel creation can't block on the editing session being ready.

Workarounds led to https://github.com/microsoft/vscode-copilot/issues/16038 and https://github.com/microsoft/vscode-copilot/issues/16060, now we have a better workaround, but the whole thing needs to be reworked so that the model and editing session setup are more aligned.