Add MCP App support to template agent

[AtrikGhosh]

Problem

Adds MCP Apps support on the agent: advertise the UI extension to MCP servers, attach mcpApp on tool results for Template UI, and expose host proxy APIs for app resources/read and tools/call.

Proposed solution

How

• On MCP initialize, advertise io.modelcontextprotocol/ui with mimeTypes: ["text/html;profile=mcp-app"].
• Filter model tool list by _meta.ui.visibility; stamp mcp_app on results (including when LangChain conversion fails but raw MCP result was captured).
• Request-scoped host proxy (mcp_host + routes): read UI resources, list tools/resources, call tools only when visibility includes app.
• Docs/config notes for enabling with Template UI (frame_src / features.mcp_apps on the UI side).

Alternatives considered

No response

Additional context

No response

[AtrikGhosh]

@NP-compete , please assign this issue to me and link it to #202

[coderabbitai]

🔗 Related PRs

#202 - feat: MCP App Implementation v3 [open]

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[AtrikGhosh]

Hi @Anish701 @nirmchan @nsaharan @NP-compete @vishnusrichand, could you please assign this issue to me? Thank you!