[codex] Support Basic roots in build and fix single-child diff traversal

[weedon-openai]

Summary

This PR combines two related updates:

1. Allow WidgetTemplate.build(...) to support Basic roots directly.
2. Fix widget diff traversal when a component's children is represented as a single object.

User impact

• Templates rooted at Basic can now be built through the standard build(...) API and used in WidgetItem without requiring a separate build path.
• Streaming widget updates are more robust for valid runtime shapes where children is a single component object, avoiding missed text delta events.

Root causes

• Root typing still restricted dynamic/strict root unions to Card and ListView, which prevented build(...) from validating Basic roots.
• diff_widget traversal only iterated children as lists; it did not recurse when children was a singular component.

Fixes

• In chatkit/widgets.py:
  • Add BasicRoot to StrictWidgetRoot.
  • Extend DynamicWidgetRoot.type to include "Basic".
  • Deprecate WidgetTemplate.build_basic(...) and direct callers to build(...).
• In chatkit/server.py:
  • Update recursive child traversal in diff_widget to handle both single-child objects and child lists.
• In tests/test_widgets.py:
  • Add coverage that WidgetTemplate.build(...) returns a DynamicWidgetRoot with type == "Basic" for basic-root templates and works in WidgetItem.
  • Add coverage that build_basic(...) emits DeprecationWarning while preserving behavior.
  • Add a regression case ensuring streaming text deltas are emitted when children is a single dynamic component object.

Validation

• make test
• Result: 122 passed