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":