Most framework integrations don't ship their own A2UI rendering tool, so the
shared snippet now guides users to inject `render_a2ui` via
`a2ui: { injectA2UITool: true }`. The LangGraph fixed-schema guide returns A2UI
operations directly from a Python tool, so the middleware auto-detects them and
tool injection is unnecessary — switched to `a2ui: {}` there with a note.