fix(a2ui): document id="root" entry-point requirement in generation guidelines

The A2UI React renderer (packages/a2ui-renderer/src/react-renderer/a2ui-react/A2uiSurface.tsx:152)
always begins rendering at the component with id="root":

    export const A2uiSurface: React.FC<{...}> = ({ surface }) => {
      // The root component always has ID 'root' and base path '/'
      return <DeferredChild surface={surface} id="root" basePath="/" />;
    };

If no component has that ID, DeferredChild falls through to its loading-
shimmer placeholder, so the surface silently renders as an empty ~30px
rectangle regardless of how many other components are on the surface.

The generation guidelines shipped to the sub-LLM (in @copilotkit/shared
and copilotkit sdk-python) never stated this requirement. Fixed-schema
demos hard-code a component with id="root" in their JSON and work; dynamic
demos relied on the LLM guessing, which it sometimes did and sometimes
didn't. The failure mode is particularly nasty: no error, no warning,
just a loading spinner that never resolves.

Adds the requirement to COMPONENT ID RULES in both the TS and Python
guideline strings. Both strings are injected into the sub-LLM's context
by A2UICatalogContext (packages/react-core) and
copilotkit.a2ui.a2ui_prompt() respectively, so every A2UI-enabled app
picks it up automatically — no per-demo change needed.

Stacked on CopilotKit#4216, which restores the same instruction to the
langgraph-python-threads demo's tool docstring (belt-and-braces until
consumers update their shared package version).

Author: maxkorp

packages/shared/src/a2ui-prompts.ts

@@ -27,6 +27,11 @@ CRITICAL: You MUST call the render_a2ui tool with ALL of these arguments:
   component names or use names not in the schema.
 
 COMPONENT ID RULES:
+- Exactly one component MUST have id="root". This is the surface's entry
+  point — the renderer begins at "root" and walks the child/children tree
+  from there. Every other component must be reachable from "root". If no
+  component has id="root", the surface renders an empty loading placeholder
+  and none of your components will be shown.
 - Every component ID must be unique within the surface.
 - A component MUST NOT reference itself as child/children. This causes a
   circular dependency error. For example, if a component has id="avatar",

sdk-python/copilotkit/a2ui.py

@@ -117,6 +117,11 @@ def render(
 - every component must have the "component" field specifying the component type (e.g. "Text", "Image", "Row", "Column", "List", "Button", etc.)
 
 COMPONENT ID RULES:
+- Exactly one component MUST have id="root". This is the surface's entry
+  point — the renderer begins at "root" and walks the child/children tree
+  from there. Every other component must be reachable from "root". If no
+  component has id="root", the surface renders an empty loading placeholder
+  and none of your components will be shown.
 - Every component ID must be unique within the surface.
 - A component MUST NOT reference itself as child/children. This causes a
   circular dependency error. For example, if a component has id="avatar",