feat(test-servers): mcp_app_demo preset + _meta on tool/resource defs (closes #1557)

Add an `mcp_app_demo` tool/resource preset — a self-contained MCP App widget
that exercises size-changed, ui/message, log notifications, and host-context
rendering with no external server. Requires plumbing an optional `_meta` field
through the composable test server's `ToolDefinition` and `ResourceDefinition`
so clients can read tool-level `_meta.ui.resourceUri` and resource-level
`_meta.ui.csp`.

- composable-test-server: optional `_meta` on ToolDefinition/ResourceDefinition,
  passed through to registerTool config and the resource content item.
- test-server-fixtures: `createMcpAppDemoTool()`, `createMcpAppDemoResource()`,
  inline `MCP_APP_DEMO_HTML` widget; wired into getDefaultServerConfig.
- preset-registry: `mcp_app_demo` tool + `mcp_app_demo_widget` resource presets.
- integration test asserting the `_meta` plumbing end-to-end.

Wave 1 of PR #1510 decomposition.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S3fTN8H3R8YV4yUGvZjYnX

test-servers/src/composable-test-server.ts

@@ -102,6 +102,8 @@ export interface ToolDefinition {
   inputSchema?: ToolInputSchema;
   /** Optional Zod object schema for tool output; when set, handler must return structuredContent. */
   outputSchema?: unknown;
+  /** Passed through to the SDK so clients can read tool-level `_meta` (e.g. `_meta.ui.resourceUri` for MCP App tools). */
+  _meta?: Record<string, unknown>;
   handler: (
     params: Record<string, unknown>,
     context?: TestServerContext,
@@ -123,6 +125,8 @@ export interface ResourceDefinition {
   description?: string;
   mimeType?: string;
   text?: string;
+  /** Included on the returned content item so clients can read resource-level `_meta` (e.g. `_meta.ui.csp` for MCP App UI resources). */
+  _meta?: Record<string, unknown>;
 }
 
 export interface PromptDefinition {
@@ -500,6 +504,7 @@ export function createMcpServer(config: ServerConfig): McpServer {
             ...(tool.outputSchema != null && {
               outputSchema: tool.outputSchema as AnySchema,
             }),
+            ...(tool._meta != null && { _meta: tool._meta }),
           },
           async (args, extra) => {
             const result = await tool.handler(
@@ -565,6 +570,7 @@ export function createMcpServer(config: ServerConfig): McpServer {
                   uri: resource.uri,
                   mimeType: resource.mimeType || "text/plain",
                   text: resource.text ?? "",
+                  ...(resource._meta != null && { _meta: resource._meta }),
                 },
               ],
             };