[codex] Fix widget diff traversal for single-child nodes

[weedon-openai]

Summary

This PR fixes widget diff traversal when a component's children field is represented as a single object instead of a list.

User-visible effect

In streaming updates, some valid widget shapes could silently skip text-delta emission. The UI would then miss incremental updates and treat the widget as unchanged (or defer to later full updates), even when streaming text content changed.

Root cause

diff_widget recursively discovers streaming Text/Markdown nodes by walking children. The previous implementation assumed children was iterable as a list and iterated directly over it. When children was a single WidgetComponentBase object, recursion did not traverse that child, so no streaming node was discovered and no delta was produced.

Fix

Update traversal in chatkit/server.py so recursion handles both valid shapes:

• children as a single WidgetComponentBase
• children as a list of components

Tests

Added a regression case in tests/test_widgets.py under test_diff that uses DynamicWidgetRoot with a single-child object and asserts a widget.streaming_text.value_delta event is emitted when text grows.

Validation run:

• PYTHONPATH=. uv run pytest tests/test_widgets.py (passes)