[MCP] Add the 3D geometry MCP App example and explorer integration

[josemontespg]

Summary

Adds one MCP App example, catalogs/mcp/v1/examples/mcp-app-3d-geometry.json, validates it against the v1.0 basic + MCP catalogs in specification/scripts/validate.py, and integrates @a2ui/catalog-mcp into the Lit, Angular, and React Explorer applications under a dedicated mcp/ directory with a client-side MockMcpServer.

The example is a 60fps canvas 3D renderer, the kind of UI the Basic Catalog cannot express, and it exercises the four things an MCP App adds on top of A2UI: two-way data.paths binding (a sibling TextField preset and the app's camera telemetry), ui/update-model-context publishing the scene state to /scene3d/modelContext, tools/call (capture_3d_snapshot), and responsive resizing through ui/notifications/host-context-changed (containerDimensions).

The example creates its surface with the basic catalog and sets catalogId on the McpApp component alone, so the Explorers register mcpCatalog next to the basic catalog without composing a merged catalog.

Changes

• catalogs/mcp/v1/examples/mcp-app-3d-geometry.json: the example, with its description documenting the MCP features it showcases; catalogs/mcp/README.md lists it.
• specification/scripts/validate.py: Add multi-catalog merging (merge_catalogs) and the mcp validation entry (catalogs/mcp/v1/catalog.json + catalogs/basic/v1/catalog.json).
• renderers/{lit,react}/a2ui_explorer: Register mcpCatalog with the MessageProcessor; renderers/angular/a2ui_explorer: McpDemoCatalog adapts mcpCatalog to AngularCatalog.
• renderers/{lit,angular,react}/a2ui_explorer/src/{,app/}mcp/: Isolate all MCP Explorer setup into mcp/mock-mcp-server.ts (client-side MockMcpServer implementing McpToolClient with simulated network latency for tools/list, resources/read, and tools/call), mcp/mcp-app-resizer.ts (window resize handles), mcp/mcp-tool-actions.ts (mockMcpServer and handleMcpToolAction) and an export-only mcp/index.ts. The MCP catalog is made of universal Web Components, so the Angular Explorer renders it only with useUniversalComponents: true (the MCP spec opts in); tool calls are routed through handleMcpToolAction as A2UI actions.
• typescript/catalogs/mcp/tests/examples/mcp_app_3d_geometry.test.ts: End-to-end Karma test of the example through the real sandbox proxy (telemetry and model context written back by the app); renderers/{lit,react,angular}/a2ui_explorer each get an example test driven through the bound preset field.

Verification

• python3 specification/scripts/validate.py
• yarn workspace @a2ui/catalog-mcp test
• yarn workspace @a2ui/{lit,react,angular}-explorer run test
• ./scripts/fix_format.sh --check && python3 scripts/fix_licenses.py --check

Related to #3157

[gemini-code-assist]

Code Review

This pull request introduces the mcp-app-order-summary.json example to the v1 catalog, showcasing an McpApp that loads and totals order items via a tool call, alongside corresponding end-to-end tests. Feedback on the changes highlights a syntax issue with nested interpolation in the total_text component expression, as well as security and robustness concerns in the inline JavaScript, such as potential prototype pollution, missing defensive checks for order items, and a lack of input sanitization.