Agent Sessions sidebar visibility is global across all windows instead of per-window

Does this issue occur when all extensions are disabled?: Yes/No

- VS Code Version: 1.120.0 Commit: 0958016b2af9f09bb4257e0df4a95e2f90590f9f (x64)
- OS Version: Ubuntu 24.04.3 LTS (x86_64), kernel 7.0.0-28-generic

Steps to Reproduce:

1. Open two VS Code windows.
2. Toggle the Agent Sessions sidebar in window A (via the command `agentSessions.toggleAgentSessionsSidebar`, or its show/hide button).
3. Observe that it also toggles in window B.

### Describe the bug

Toggling the Agent Sessions sidebar (`agentSessions.toggleAgentSessionsSidebar` / `agentSessions.showAgentSessionsSidebar` / `agentSessions.hideAgentSessionsSidebar`) in one window also toggles it in every other open window.

Its visibility appears to be stored as global user state rather than per-window/per-workspace state, unlike normal side-bar/panel visibility which is per-window.

**Expected:** Showing/hiding the Agent Sessions sidebar affects only the current window, matching the behavior of other views/panels.

**Actual:** The toggle applies to all open windows simultaneously.