Add MCP Apps extension support (typed metadata, attribute, and helpers)

[Copilot]

MCP Apps is the first official MCP extension (io.modelcontextprotocol/ui), enabling servers to deliver interactive UIs inside AI clients. The C# SDK had the foundational primitives (Extensions, _meta) but no typed convenience layer, requiring manual JSON construction that is error-prone and non-discoverable.

New APIs

F1 – Constants (McpApps)

• McpApps.ResourceMimeType → "text/html;profile=mcp-app"
• McpApps.ExtensionId → "io.modelcontextprotocol/ui"
• McpApps.ResourceUriMetaKey → "ui/resourceUri" (legacy backward-compat key)

F2 – Typed metadata models

• McpUiToolMeta – ResourceUri, Visibility
• McpUiToolVisibility – Model/App string constants
• McpUiResourceMeta – Csp, Permissions, Domain, PrefersBorder
• McpUiResourceCsp – ConnectDomains, ResourceDomains, FrameDomains, BaseUris
• McpUiResourcePermissions – Allow
• McpUiClientCapabilities – MimeTypes

F3 – Client capability helper

McpUiClientCapabilities? caps = McpApps.GetUiCapability(clientCapabilities);
if (caps?.MimeTypes?.Contains(McpApps.ResourceMimeType) is true) { ... }

F6 – [McpAppUi] attribute (declarative path)

[McpServerTool]
[McpAppUi(ResourceUri = "ui://weather/view.html")]
[Description("Get current weather for a location")]
public string GetWeather(string location) => ...;

Takes precedence over any [McpMeta("ui", ...)] on the same method. Populates both _meta.ui (structured) and the legacy _meta["ui/resourceUri"] flat key automatically.

F7 – McpServerToolCreateOptions.AppUi (programmatic path)

var tool = McpServerTool.Create(handler, new McpServerToolCreateOptions
{
    AppUi = new McpUiToolMeta
    {
        ResourceUri = "ui://weather/view.html",
        Visibility = [McpUiToolVisibility.Model, McpUiToolVisibility.App]
    }
});

Explicit Meta entries take precedence over AppUi; AppUi takes precedence over McpMetaAttribute.

Notes

• All new APIs are [Experimental(MCPEXP001)] — same diagnostic as Extensions and Tasks.
• New types registered with McpJsonUtilities.JsonContext for Native AOT compatibility.
• F4 (RegisterAppTool) and F5 (RegisterAppResource) are not included; the C# DI-based registration model makes F6/F7 the idiomatic equivalents.