test: cover the structured-output cap and the list_items showcase end to end

Two gaps from the Copilot review on #1975:

- The `Large` story asserted nothing, so dropping the section's `mah` (or
  applying it to the wrong element) would still pass CI. Its play function
  now measures the real ScrollArea viewport: the visible height stays at
  the cap and the overflow is scrollable rather than clipped.

- Nothing exercised the `list_items` fixture, so a preset-registration
  typo would leave the documented showcase broken with a green suite. A
  new integration test loads `structured-output-http.json` through
  loadConfig/resolveConfig — covering the file, the registry entry, and
  the fixture — connects, calls the tool, and asserts both the summary
  block and the nested structuredContent.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SW1p8E2uiyyLx4RKwrwSrt

clients/web/src/components/groups/StructuredOutputPanel/StructuredOutputPanel.stories.tsx

@@ -59,10 +59,29 @@ export const Flat: Story = {
   },
 };
 
+// A payload taller than the section's cap scrolls inside it rather than
+// pushing the rest of the result panel down. Asserted on the real geometry,
+// so dropping `mah` (or moving it to the wrong element) fails here.
 export const Large: Story = {
   args: {
     structuredContent: large,
   },
+  play: async ({ canvasElement }) => {
+    await waitFor(() =>
+      expect(canvasElement.textContent).toContain('"Row 60"'),
+    );
+    const viewport = canvasElement.querySelector(
+      ".mantine-ScrollArea-viewport",
+    );
+    if (!(viewport instanceof HTMLElement)) {
+      throw new Error("scroll viewport not found");
+    }
+    // Capped: the visible height stays at the section's `mah`, well under the
+    // payload's natural height…
+    expect(viewport.clientHeight).toBeLessThanOrEqual(400);
+    // …and the overflow is reachable by scrolling rather than clipped away.
+    expect(viewport.scrollHeight).toBeGreaterThan(viewport.clientHeight);
+  },
 };
 
 // Collapsing hides the payload; expanding brings it back.