userAction is encoded as text not as data in A2A envelope

[ymmt2005]

Describe the bug
A2UI A2A extension defines the encoding of A2UI messages as follows.
https://a2ui.org/specification/v0.8-a2a-extension/#data-encoding

The data field of the DataPart contains the A2UI JSON message (e.g., surfaceUpdate, userAction).

However, genui (and genui_a2ui?) encodes an A2UI message as a TextPart, serializing it to a JSON string.

[jacobsimionato]

Thank you, @ymmt2005 ! We should fix this non-compliance with the specification.

In practice, we're looking at switching to A2UI v0.9 very soon and I believe this is implemented properly there, so we should just verify this.

genui/packages/genui_a2ui/lib/src/a2ui_agent_connector.dart

Lines 104 to 111 in 172d137

	} else if (part.isUiInteractionPart) {
	final genui.UiInteractionPart uiPart = part.asUiInteractionPart!;
	try {
	final Object? json = jsonDecode(uiPart.interaction);
	if (json is Map<String, Object?>) {
	return Part.data(data: json);
	}
	return Part.text(text: uiPart.interaction);

genui/packages/genui/lib/src/model/parts/ui.dart

Line 119 in 172d137

factory UiInteractionPart.fromDataPart(DataPart part) {