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/preset-registry.ts

@@ -40,6 +40,8 @@ import {
   createOptionalTaskTool,
   createForbiddenTaskTool,
   createImmediateReturnTaskTool,
+  createMcpAppDemoTool,
+  createMcpAppDemoResource,
   createArchitectureResource,
   createTestCwdResource,
   createTestEnvResource,
@@ -153,6 +155,8 @@ function resolveToolPreset(
         get("name") as string | undefined,
         Number(get("delayMs")) || undefined,
       );
+    case "mcp_app_demo":
+      return createMcpAppDemoTool();
     default:
       throw new Error(`Unknown tool preset: ${name}`);
   }
@@ -175,6 +179,8 @@ function resolveResourcePreset(
       return createTestArgvResource();
     case "numbered_resources":
       return createNumberedResources(Number(get("count")) || 3);
+    case "mcp_app_demo_widget":
+      return createMcpAppDemoResource();
     default:
       throw new Error(`Unknown resource preset: ${name}`);
   }