Agents mode: sessions get "stuck" and session history renders blank when switching between sessions

## Type: Bug

### Area
Chat / Copilot **Agents mode** (agent sessions)

### Summary
Working in Agents mode has noticeably more friction than the rest of the VS Code experience. Two problems in particular are recurring and disruptive:

1. **Sessions get "stuck".** A session occasionally stops making progress / stops responding and appears frozen, even though nothing is obviously wrong.
2. **Session history renders blank when navigating between sessions.** When I switch to another session and then return to a previous one, the conversation history for that previous session sometimes shows up **blank / not visible** — even though the underlying **context is still intact** (the agent still "remembers" everything and continues correctly once I interact with it). It's purely a rendering/UI problem: the transcript isn't drawn, but the state is there.

### Steps to reproduce
1. Open Agents mode and start a session; let it run for a while (multiple turns / tool calls).
2. Open or switch to a second (different) session.
3. Switch back to the first session.
4. **Observe:** the first session's history pane is sometimes empty / not rendered, even though the session's context is preserved and the agent can continue as if the history were present.

Separately:
1. Run a longer agent session with several tool calls.
2. **Observe:** the session sometimes becomes "stuck" and stops progressing.

### Expected behavior
- Switching away from and back to a session should always re-render its full conversation history (parity with reopening a normal editor/panel).
- Sessions should not get stuck; if a background operation stalls there should be a clear indication and a way to recover.

### Actual behavior
- Returning to a previous session can show a blank history even though context is retained.
- Sessions intermittently get stuck with no clear feedback or recovery path.

### Impact
These issues make Agents mode feel less polished and more error-prone than the regular VS Code experience. The blank-history-on-navigation issue is especially confusing because it looks like work was lost when it actually wasn't.

### Additional notes
- The blank history appears to be a **UI/rendering** issue rather than data loss — the context is still present and the agent behaves correctly after the switch.
- Would help to have: (a) a reliable re-render of session transcripts on focus/navigation, and (b) a visible "session is busy / stalled" state with a cancel/restart affordance.

### Environment

- VS Code version:
- Copilot / Copilot Chat extension version:
- OS: Windows