fix: render a tool result's structuredContent in the Tools screen (#1908)

A tool declaring an `outputSchema` returns its real payload in
`structuredContent`; the `content[]` blocks usually only summarize it. v1
rendered that payload as its own inspectable JSON section — v2's
ToolResultPanel only ever walked `result.content`, so the payload was
dropped with no label, section, or hint that it existed.

Adds a `StructuredOutputPanel` group: a collapsible "Structured Output"
box (bordered like the existing "Resource Links" box) rendering the
payload as pretty-printed, syntax-highlighted, copyable JSON, bounded so
a large payload scrolls within itself. ToolResultPanel renders it below
the content blocks — and also when `content` is empty (where "No results
yet" was simply wrong) and below the alert on an error result, so it is
never silently dropped.

Adds a `list_items` test-server preset (nested payload, the issue's repro
shape) and a `structured-output-http.json` showcase config, documented in
the README.

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

test-servers/src/preset-registry.ts

@@ -34,6 +34,7 @@ import {
   createGetAnnotatedMessageTool,
   createGetTempTool,
   createGetTempExtraTool,
+  createListItemsTool,
   createAddResourceTool,
   createRemoveResourceTool,
   createAddToolTool,
@@ -158,6 +159,8 @@ function resolveToolPreset(
       return createGetTempTool();
     case "get_temp_extra":
       return createGetTempExtraTool();
+    case "list_items":
+      return createListItemsTool();
     case "add_resource":
       return createAddResourceTool();
     case "remove_resource":

test-servers/src/test-server-fixtures.ts

@@ -1048,6 +1048,48 @@ export function createGetTempTool(): ToolDefinition {
   };
 }
 
+/** Output schema for list_items: a nested list of items plus a total. */
+const ListItemsOutputSchema = z.object({
+  items: z
+    .array(
+      z.object({
+        id: z.number().describe("Item id"),
+        name: z.string().describe("Item name"),
+        tags: z.array(z.string()).describe("Item tags"),
+      }),
+    )
+    .describe("The items"),
+  total: z.number().describe("Number of items"),
+});
+
+/**
+ * Create a "list_items" tool returning a short `content` summary alongside a
+ * DEEPLY NESTED `structuredContent` (objects inside arrays inside an object) —
+ * the shape from #1908, where the structured payload carries everything the
+ * text block only summarizes. Exercises the Structured Output section of the
+ * Tools screen result panel, which `get_temp`'s flat three-key object does not.
+ */
+export function createListItemsTool(): ToolDefinition {
+  return {
+    name: "list_items",
+    description: "Returns a list of items with nested structured output",
+    inputSchema: {},
+    outputSchema: ListItemsOutputSchema,
+    handler: async () => {
+      const items = [
+        { id: 1, name: "Item A", tags: ["foo", "bar"] },
+        { id: 2, name: "Item B", tags: ["baz"] },
+      ];
+      return {
+        content: [
+          { type: "text" as const, text: `Found ${items.length} items.` },
+        ],
+        structuredContent: { items, total: items.length },
+      };
+    },
+  };
+}
+
 /**
  * Create a "get_temp_extra" tool that declares the same output schema as
  * get_temp but returns an EXTRA, undeclared property in structuredContent.