Transfer chat session only works on new workspace

[roblourens]

• From a workspace with files in it, ask Copilot Chat to create a new project
• Go through the new workspace flow- it asks you to open an empty folder
• Pick an empty folder that has never been opened in vscode before
• New workspace flow works, continues setting up project 👍
• But, if you pick an empty folder that you have used before, chat view is broken and empty

When switching to use the ChatSessionStore, I didn't store transferred chat sessions here, they stay in the storage service. I thought this worked by falling back to the migrated session (see migrateDataIfNeeded). But this only happens the first time the workspace is opened, otherwise the session isn't transferred at all.