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.

scripts/generate-schemas.ts

@@ -70,8 +70,10 @@ const JSON_SCHEMA_OUTPUT_FILE = join(GENERATED_DIR, "schema.json");
 const EXTERNAL_TYPE_SCHEMAS = [
   "ContentBlockSchema",
   "CallToolResultSchema",
+  "EmbeddedResourceSchema",
   "ImplementationSchema",
   "RequestIdSchema",
+  "ResourceLinkSchema",
   "ToolSchema",
 ];