Session disappears from pinned list, then reappears randomly

### Description

Active sessions intermittently disappear from the pinned list (count 3→2), reappear on selecting another session.

What happens: With 3 pinned chat sessions, the pinned count intermittently drops to 2 and one session vanishes from the list. It reappears if I select any other session, after which it's selectable again. The session itself is unaffected — no lost state, no interruption.

Key signal: The disappearing session is always one of the two currently-active sessions (ones actively streaming/running), never the idle one. Which of the two drops appears random, correlating with periods when both are updating.

Suspected cause: A render/reconcile race in the Chat Sessions tree triggered by frequent status updates on active sessions — a mid-update snapshot momentarily omits the item being updated. Idle sessions never churn, so they never drop.

Repro:

Pin 3 chat sessions.
Have 2 of them actively running/streaming at once.
Watch the pinned list — count intermittently shows 2; an active session disappears.
Select any other session — the missing one reappears.
Workaround: Selecting another session, or reloading the window, restores it.

### Issue Details

Item|Value
---|---
Issue Category|Bug
Target|Visual Studio Code
VS Code Version|Code 1.128.0 (fc3def6774c76082adf699d366f31a557ce5573f, 2026-07-07T15:14:24-07:00)
OS Version|Windows_NT x64 10.0.26200

### Additional Issue Data

[issue-data.md](https://github.com/user-attachments/files/29930890/issue-data.md)