Whole-dashboard snapshot/screenshot support in Preset MCP server

What is the problem or goal you're trying to solve or accomplish?

The Preset MCP server only exposes preview/snapshot capability at the chart level (get_chart_preview). There's no way to get a snapshot of an entire dashboard in one call. This is a regression from the classic Superset REST API, which supports whole-dashboard screenshots via GET /api/v1/dashboard/{id}/thumbnail. Customers migrating workflows to MCP lose that capability, they have no single-call way to capture what a full dashboard looks like.

How are you solving it currently?

There's no direct equivalent through MCP. The only workaround is to enumerate a dashboard's charts (via get_dashboard_info / get_dashboard_layout) and call get_chart_preview on each one individually, then manually stitch those together, there's no single composed image or output representing the dashboard as a whole.

What is your recommended solution?

Add a get_dashboard_preview (or get_dashboard_screenshot) tool to preset-mcp that either:

1. Proxies Superset's existing dashboard thumbnail/screenshot endpoint and returns a rendered image (base64 or URL),
   reusing the async screenshot/cache-key flow that endpoint already relies on, or

2. If a true rendered screenshot isn't feasible over the MCP transport, composes the individual chart previews (using
   the existing get_dashboard_layout + get_chart_preview internals) into one combined dashboard-level preview.