McpAdapter: declare MCP Apps UI capability on client init

[bokelley]

Problem

McpAdapter initializes the MCP client without declaring UI/Apps capabilities:

this.mcpClient = new Client({
  name: 'scope3-sdk',
  version: SDK_VERSION,
}, {
  capabilities: {
    tools: {},
  },
});

Without declaring the io.modelcontextprotocol/ui extension capability, the MCP server has no way to know that this client supports MCP Apps. Servers that gate UI metadata (like _meta.ui.resourceUri) behind capability negotiation will not include it in responses to this client.

Proposed Solution

Add the MCP Apps extension capability to the client initialization:

this.mcpClient = new Client({
  name: 'scope3-sdk',
  version: SDK_VERSION,
}, {
  capabilities: {
    tools: {},
  },
});

// Declare MCP Apps support
// Uses @mcp-ui/client's UI_EXTENSION_CAPABILITIES or manual declaration

The exact capability key depends on the current MCP Apps spec — likely io.modelcontextprotocol/ui with config { mimeTypes: ["text/html;profile=mcp-app"] }.

Related

Depends on #128 — declaring the capability is only useful if consumers can actually access the raw CallToolResult and MCP App resources.