[codex] Support Basic roots in WidgetTemplate.build

[weedon-openai]

Summary

This PR enables Basic roots in the standard widget build path and deprecates the transitional WidgetTemplate.build_basic helper.

User-visible effect

• Developers can now call WidgetTemplate.build(...) for templates whose root is Basic
• Developers can now use BasicRoot type widgets with WidgetItem.

Implementation

• Add BasicRoot to StrictWidgetRoot.
• Expand DynamicWidgetRoot.type to include "Basic".
• Keep WidgetTemplate.build_basic(...) for compatibility, but mark it deprecated with guidance to use build(...).
• Add tests covering:
  • build(...) returns a DynamicWidgetRoot with type == "Basic" for a basic-root template.
  • resulting widgets can be used in WidgetItem.
  • build_basic(...) emits DeprecationWarning and still produces the expected BasicRoot payload.