WebviewPanel: expose tooltip property on panel tab (like editor tabs for files)

## Problem

`vscode.window.createWebviewPanel` lets extensions set a `title` and `iconPath` on the tab, but there is no way to set a tooltip on the tab label.

For editor tabs opened from file URIs, VS Code automatically shows the full file path on hover. There is no equivalent for WebView panels: the only thing visible on hover is the truncated title string itself.

## Use case

Our extension (Google Earth Engine) opens asset preview panels whose tab title is the short asset name (e.g. `my_image`). The full Earth Engine asset path (`projects/my-project/assets/folder/my_image`) is important context that users need — particularly when several panels with similar short names are open side-by-side.

We would like to display the short name as the tab title and the full path as the hover tooltip, exactly mirroring what VS Code does for file editor tabs.

Alternatively, a description property (already present on `TreeItem`, `StatusBarItem`, etc.) shown as secondary text or tooltip on the tab would also address the need.