feat: harden workflow gates and expose token usage

frontend/src/viewers/ArtifactView.test.tsx

@@ -21,6 +21,11 @@ test("sql artifact accepts a { content } wrapper", async () => {
   await screen.findByTestId("hl");
 });
 
+test("an empty artifact shows an explicit fallback instead of a blank panel", () => {
+  render(<ArtifactView artifact={{ kind: "sql", data: {} }} />);
+  expect(screen.getByText("No artifact content available.")).toBeInTheDocument();
+});
+
 test("cte_plan renders an ordered list of names", () => {
   render(<ArtifactView artifact={{ kind: "cte_plan", data: { names: ["a_cte", "b_cte"] } }} />);
   expect(screen.getByText("a_cte")).toBeInTheDocument();

frontend/src/viewers/ArtifactView.tsx

@@ -135,6 +135,9 @@ function StructuredValue({ value, depth = 0 }: { value: unknown; depth?: number
     );
   }
   const entries = Object.entries(value as Record<string, unknown>);
+  if (entries.length === 0) {
+    return <p className="text-sm text-muted-foreground">No artifact content available.</p>;
+  }
   return (
     <div className="flex flex-col">
       {entries.map(([k, v], i) => (