Agents window: let us center the bottom panel instead of it always spanning full width

## The problem

In the Agents window, the bottom panel always spans everything to the right of the Sidebar, it is not possible to center align it, like there is in the regular VS Code instance. This causes the right sidebar to get squeezed up. It also means its not possible to use the `View: Toggle Maximized Panel` command, which instead shows the error: "Maximizing the panel is only supported when it is center aligned." You can see that happening in this image.

## What I would like

A way to center align the panel in the Agents window, so it spans the chat and editor area only and leaves the right Sidebar at full height, like I have hacked together here below.

## Why it is not possible today

The Agents window layout fixes the alignment and ignores the commands:

https://github.com/microsoft/vscode/blob/main/src/vs/sessions/browser/workbench.ts#L2661-L2667

```ts
getPanelAlignment(): PanelAlignment {
return 'justify'; // Full width panel
}

setPanelAlignment(_alignment: PanelAlignment): void {
// No-op: Panel alignment is fixed in this layout
}
```

- VS Code Version: 1.138.0
- OS Version: macOS 26.6.2 (arm64)