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/configs/structured-output-http.json

@@ -0,0 +1,15 @@
+{
+  "serverInfo": {
+    "name": "structured-output-showcase",
+    "version": "1.0.0"
+  },
+  "tools": [
+    { "preset": "list_items" },
+    { "preset": "get_temp" },
+    { "preset": "echo" }
+  ],
+  "transport": {
+    "type": "streamable-http",
+    "port": 6601
+  }
+}