Agents window should have icon in window picker

[mjbvz]

1. Open agents window
2. Use switch window

Bug

The agents window should have an icon to s it's easy to tell apart from the other vscode windows

[vs-code-engineering]

Sharing media and context from merged PR #335333 for verification:

• Add an Agents Window task to the Windows taskbar jump list
• Launch it through the existing --agents mode
• Use the distro-mixed sessions.ico asset for the jump-list task and resulting Windows taskbar button, so Stable, Insiders, and Exploration receive matching product-quality branding
• Expose each native CodeWindow icon path through generic opened-window metadata
• Have Switch Window render the icon advertised by the window, preserving existing file and workspace icons when a native icon is not provided

Screenshots

Related issues

Source PR: #335333