refactor: use MCP resource types for ui/download-file params

Replace custom {filename, content, mimeType, encoding} params with
standard MCP resource types: EmbeddedResource for inline content and
ResourceLink for host-resolved references.

New params shape:
  params: { contents: (EmbeddedResource | ResourceLink)[] }

This reuses existing MCP primitives instead of inventing new ones,
and enables hosts to show resource 'bubbles' or resolve ResourceLink
URIs via resources/read on the MCP server.

examples/integration-server/server.ts

@@ -16,6 +16,7 @@ const DIST_DIR = import.meta.filename.endsWith(".ts")
   ? path.join(import.meta.dirname, "dist")
   : import.meta.dirname;
 const RESOURCE_URI = "ui://get-time/mcp-app.html";
+const SAMPLE_DOWNLOAD_URI = "resource:///sample-report.txt";
 
 /**
  * Creates a new MCP server instance with tools and resources registered.
@@ -70,5 +71,28 @@ export function createServer(): McpServer {
     },
   );
 
+  // Sample downloadable resource — used to demo ResourceLink in ui/download-file
+  server.resource(
+    SAMPLE_DOWNLOAD_URI,
+    SAMPLE_DOWNLOAD_URI,
+    {
+      mimeType: "text/plain",
+    },
+    async (): Promise<ReadResourceResult> => {
+      const content = [
+        "Integration Test Server — Sample Report",
+        `Generated: ${new Date().toISOString()}`,
+        "",
+        "This file was downloaded via MCP ResourceLink.",
+        "The host resolved it by calling resources/read on the server.",
+      ].join("\n");
+      return {
+        contents: [
+          { uri: SAMPLE_DOWNLOAD_URI, mimeType: "text/plain", text: content },
+        ],
+      };
+    },
+  );
+
   return server;
 }