Use content instead of structuredContent in examples

Not all MCP clients support structuredContent in MCP tool results, so
examples now return data as JSON in content and parse it client-side.

Pattern used:
- Server: content: [{ type: "text", text: JSON.stringify(data) }]
- Client: JSON.parse(result.content!.filter(...).map(...).join(""))

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

examples/cohort-heatmap-server/src/mcp-app.tsx

@@ -103,7 +103,13 @@ function CohortHeatmapInner({ app }: { app: App }) {
           maxPeriods: 12,
         },
       });
-      setData(result.structuredContent as unknown as CohortData);
+      const text = result
+        .content!.filter(
+          (c): c is { type: "text"; text: string } => c.type === "text",
+        )
+        .map((c) => c.text)
+        .join("");
+      setData(JSON.parse(text) as CohortData);
     } catch (e) {
       console.error("Failed to fetch cohort data:", e);
     } finally {