[Proposal] Add support for UI Resource Templates in SEP-1865

Motivation

The current draft of SEP-1865 primarily focuses on static UI resources. However, the core Model Context Protocol already defines Resource Templates for dynamic URI resolution. If we support ResourceTemplates it would make Server-Side Rendering (SSR) for MCP Apps very-very powerful.

Proposed Specification Change

We should explicitly allow _meta.ui.resourceUri to be an MCP Resource Template (URI Template - RFC 6570).

1. Alignment: Leverage the existing MCP mechanism where tools can reference template-based URIs.
2. SSR Logic: Instead of the UI performing client-side data fetching, the request goes to the server for a specific resource and server the server generates the entire page (good old simple SSR).
3. Efficiency: This allows the server to return pre-rendered HTML specifically for this resource.

Example

1. Tool Definition (on Server)
The server registers a tool with a template URI:

{
  "name": "get_pizza_details",
  "_meta": {
    "ui": { "resourceUri": "ui://pizzas/{pizzaId}/details" }
  }
}

2. Tool Response
The tool returns the specific ID in the structuredContent:

{
  "structuredContent": { "pizzaId": "margherita-123" }
}

3. Host Action
The Host extracts the pizzaId from the tool response and reads the resource template
ui://pizzas/margherita-123/details.

4. Server
Generates the details page for margherita-123.

Goal

To simplify the developer experience by allowing "0-JS" or low-JS apps, moving the logic from the sandboxed client back to the server.

Checkout this PR to see this concretely: https://github.com/modelcontextprotocol/ext-apps/pull/296

Start by reading the current SEP-1865 draft and the concrete approach in PR #296. Compare the proposed Resource Template flow with the existing MCP Resource Template mechanism and identify the specification decisions still needed. Done means the proposal is resolved and any agreed SEP-1865 changes are documented.