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.

clients/web/src/test/integration/mcp/inspectorClient.test.ts

@@ -1,4 +1,5 @@
 import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { resolve } from "node:path";
 import * as z from "zod/v4";
 import { InspectorClient } from "@inspector/core/mcp/inspectorClient.js";
 import {
@@ -45,6 +46,8 @@ import {
   createAddResourceTool,
   createAddToolTool,
   createAddPromptTool,
+  loadConfig,
+  resolveConfig,
 } from "@modelcontextprotocol/inspector-test-server";
 import type {
   MessageEntry,
@@ -1214,6 +1217,52 @@ describe("InspectorClient", () => {
     });
   });
 
+  describe("Structured output showcase config (#1908)", () => {
+    // Drives the shipped `structured-output-http.json` end to end — the file
+    // itself, the `list_items` preset registration, and the fixture's nested
+    // payload. A typo in any of the three fails here rather than leaving the
+    // documented showcase quietly broken.
+    const showcaseConfigPath = resolve(
+      import.meta.dirname,
+      "../../../../../../test-servers/configs/structured-output-http.json",
+    );
+
+    it("serves list_items with a summary block and a nested structuredContent", async () => {
+      server = createTestServerHttp(
+        resolveConfig(loadConfig(showcaseConfigPath)),
+      );
+      await server.start();
+
+      client = new InspectorClient(
+        { type: "streamable-http", url: server.url },
+        { environment: { transport: createTransportNode } },
+      );
+      await client.connect();
+
+      const tool = await getTool(client, "list_items");
+      expect(tool.outputSchema).toBeDefined();
+
+      const result = await client.callTool(tool, {});
+      expect(result.success).toBe(true);
+
+      // The text block only summarizes — the payload is the structured half.
+      const content = result.result!.content as Array<{
+        type: string;
+        text?: string;
+      }>;
+      expect(content[0].type).toBe("text");
+      expect(content[0].text).toBe("Found 2 items.");
+
+      expect(result.result!.structuredContent).toEqual({
+        items: [
+          { id: 1, name: "Item A", tags: ["foo", "bar"] },
+          { id: 2, name: "Item B", tags: ["baz"] },
+        ],
+        total: 2,
+      });
+    });
+  });
+
   describe("Default metadata (server-wide _meta)", () => {
     function metaOf(req: { message: unknown }): Record<string, unknown> {
       const params = (req.message as { params?: { _meta?: unknown } }).params;