[MCP] feat(samples): add the Kanban board showcase for agent and MCP App communication

[josemontespg]

Summary

Adds a Kanban board sample under samples/community/mcp/a2ui-kanban/ that connects a Node MCP server, a Python ADK + A2A agent, and a Lit chat host through @a2ui/catalog-mcp.

The sample exercises four communication paths around the McpApp component: the agent mutates the board via MCP server tools and pushes /kanban/board updates to the app (updateDataModel to ui/notifications/data-model-update); the app publishes selection, filter, and column WIP state to /kanban/modelContext (ui/update-model-context) so the agent can answer questions about the live view; card drags write /kanban/board (ui/notifications/data-model-change) so sibling Text counters recompute client-side without an agent turn; and card drops (tools/call + ui/message) and split requests (ui/message) dispatch a2ui.mcpAppMessage actions that the chat host forwards to the agent.

Live demo: http://josemontesp.c.googlers.com:5173/

Changes

• samples/community/mcp/a2ui-kanban/mcp-server/: Streamable HTTP MCP server exposing list_board, move_task, create_task, assign_task, and open_board, plus the @modelcontextprotocol/ext-apps board UI bundled into the ui://kanban/board resource (text/html;profile=mcp-app) and unit/HTTP tests.
• samples/community/mcp/a2ui-kanban/agent/: Python ADK + A2A agent (and a deterministic KANBAN_AGENT=scripted fallback for tests and keyless runs) that connects to the MCP server and emits A2UI v1.0 surface and data-model messages, with pytest coverage.
• samples/community/mcp/a2ui-kanban/client/: Lit chat host registering basicCatalog and mcpCatalog, connecting callMcpTool to the MCP server, deriving /kanban/summary/* from /kanban/board for sibling Text components, and testing all four channels with Vitest and Playwright E2E tests.
• samples/community/mcp/a2ui-kanban/README.md, samples/community/README.md, samples/community/package.json, samples/community/yarn.lock: Three-terminal setup instructions, walkthrough storyline, and community workspace registration.
• docs/public/guides/mcp-apps-in-a2ui.md: Adds the Kanban showcase as Sample 3.

Verification

• Ran yarn test in samples/community/mcp/a2ui-kanban/mcp-server (13 node:test server/tool tests + 20 Vitest app tests passing), yarn test in samples/community/mcp/a2ui-kanban/client (9 Vitest host tests passing), yarn test:e2e in samples/community/mcp/a2ui-kanban/client (6 Playwright E2E CUJ + WCAG AA contrast tests passing), and uv run pytest in samples/community/mcp/a2ui-kanban/agent (11 tests passing).
• Built a2ui-kanban-mcp-server and a2ui-kanban-client from samples/community, ran uv sync --no-install-project, verified formatting and license headers, and ran the three services end-to-end (http://josemontesp.c.googlers.com:5173/); full community_code.yml workflow runs in CI.

Related to #3157

[gemini-code-assist]

Code Review

This pull request introduces a new Kanban Board Showcase sample application, demonstrating bidirectional communication between a Python agent, a sandboxed MCP App, and sibling A2UI components. The feedback focuses on enhancing the robustness and error handling of the newly added code. Key recommendations include wrapping server tool calls in try-catch blocks to safely roll back optimistic UI updates on failure, and adding defensive type checks in the agent executor, MCP client, and chat host to prevent potential runtime crashes from malformed or unexpected data payloads.