Copilot local chat session disappears from Sessions after Windows restart despite intact JSONL and indexes

### Environment

* VS Code: 1.136.2
* GitHub Copilot Chat: 0.64.1
* OS: Windows
* Local workspace

### Steps to Reproduce

1. Have an existing Copilot Chat session open in a local workspace.
2. Windows restarts unexpectedly.
3. Reopen VS Code and the same workspace.
4. Open the Sessions view.

### Expected

The existing chat session should still be listed and accessible.

### Actual

The session is missing from the Sessions UI and cannot be reopened.

However, the persisted session data still appears intact:

* `chatSessions/.jsonl` exists and contains the conversation.
* `chat.ChatSessionStore.index` contains the session with `isEmpty: false`.
* `agentSessions.model.cache` contains the corresponding local session.
* The session is not marked as archived in `agentSessions.state.cache`.
* **Reindex Sessions** does not restore it.

Other sessions still appear normally.

ps.Toggling Chat: Disable AI Features on/off (#297800) does not restore the missing session.