chore: add changeset for MCP structuredContent feature

- Patch version bump for @openai/agents-core
- Documents new useStructuredContent option and CallToolResult changes
- Maintains backward compatibility by default

Author: boice-sealint

.changeset/mcp-structured-content.md

@@ -0,0 +1,9 @@
+---
+'@openai/agents-core': patch
+---
+
+feat(mcp): add structuredContent support behind `useStructuredContent`; return full CallToolResult from `callTool`
+
+- `MCPServer#callTool` now returns the full `CallToolResult` (was `content[]`), exposing optional `structuredContent`.
+- Add `useStructuredContent` option to MCP servers (stdio/streamable-http/SSE), default `false` to avoid duplicate data by default.
+- When enabled, function tool outputs include `structuredContent` (or return it alone when no `content`).