feat: MCP Apps (ext-apps) UI rendering support

[wesleysanjose]

Summary

• Preserve _meta from MCP CallToolResult through the entire backend pipeline (mcp-client → CommonToolResult → mcp-gateway.utils → chat-mcp-client)
• Add readResource() to McpClient for fetching ui:// resources via the MCP SDK resources/read method
• Render MCP App UI in sandboxed iframes in chat tool output using the srcdoc attribute
• Support postMessage-based iframe resize per the ext-apps spec

How it works

1. When an MCP tool returns _meta.ui.resourceUri, the backend immediately reads the HTML resource from the same MCP server connection
2. The HTML is wrapped alongside _mcpMeta in the tool result JSON that flows to the frontend
3. The frontend ToolOutput component detects the _appHtml field and renders it in a sandboxed iframe
4. The iframe only has allow-scripts — no access to parent page, cookies, or navigation

Files changed

File	Change
common-llm-format.ts	Add _meta field to CommonToolResult
mcp-client.ts	Preserve _meta in createSuccessResult, add readResource() method
mcp-gateway.utils.ts	Forward _meta in MCP Gateway tool call responses
chat-mcp-client.ts	Wrap _mcpMeta + _appHtml in tool output, read ui:// resources
tool.tsx	Extract MCP metadata, render McpAppFrame with srcdoc
mcp-client.test.ts	Add 5 new tests for _meta preservation and readResource
platform-chat.md	Document MCP Apps UI feature

Test plan

• All 2402 backend tests pass (36 mcp-client, 18 mcp-gateway)
• TypeScript type-check passes (backend + frontend)
• Biome lint passes
• Manual test with an MCP server that returns _meta.ui.resourceUri

Closes #1301

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📊 Reputation Summary

User	Rep	Pull Requests	Activity	Assigned	Core Reactions
joeyorlando	⚡ 2162	87✅ 4🔄 9❌	100 issues, 50 comments	6	—
wesleysanjose	⚡ 3	0✅ 1🔄 0❌	0 issues, 1 comments	0	—
CLAassistant	⚡ 0	0✅ 0🔄 0❌	0 issues, 50 comments	0	—

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[joeyorlando]

hi there @wesleysanjose 👋 do you have a demo video showing how this works with two MCP servers which return "MCP-UI compatible responses"?