Implement v0.9 spec and "Bring Your Own LLM" API

[gspencergoog]

Description

This implements the A2UI v0.9 spec and reorganizes the API around a "Bring your own LLM" strategy which eliminates ContentGenerators and their associated packages.

[gemini-code-assist]

Code Review

This pull request primarily focuses on migrating the GenUI framework and its examples to A2UI protocol version 0.9, which involves significant renaming and restructuring of core components and message types. Key changes include replacing BeginRendering with CreateSurface, SurfaceUpdate with UpdateComponents, and DataModelUpdate with UpdateDataModel. The component structure in JSON payloads has been flattened, moving from nested objects like {"Text": {...}} to a more direct {"component": "Text", ...} format. The GenUiSurface widget's host parameter has been renamed to genUiContext across the codebase. The ContentGenerator abstraction has been replaced by a new AiClient interface and GenUiController for managing LLM interactions and UI state, leading to the removal of Firebase-specific AI integrations and the introduction of dartantic_ai in the simple_chat example. Platform-specific plugin registrations were updated to remove Firebase and add url_launcher dependencies. Review comments highlight a bug in the _generate method of GoogleGenerativeAiClient causing duplicate error emissions, a concern about the custom JSON schema validator in UiDefinition being prone to errors and incomplete, and an ambiguous handling of null values in list updates within DataModel._updateValue. Additionally, the _handleChatMessage method in ChatSession was noted for only logging events instead of triggering further LLM requests, making the example incomplete.

[gemini-code-assist]

The _generate method catches exceptions, adds them to the _errorController, and then re-throws them. The calling method, sendRequest, also has a try-catch block that will catch this re-thrown exception and add it to the same _errorController. This will result in duplicate error events being emitted for a single failure.

The migration plan correctly identifies this as BUG-004.

Suggested change

	genUiLogger.severe('Error from service.generateContent', e, st);
	_errorController.add(ContentGeneratorError(e, st));
	_errorController.add(e);
	rethrow;
	_errorController.add(e);
	return;

[gemini-code-assist]

The new validate method implements a custom JSON schema validator from scratch. Re-implementing a validator is complex and prone to errors. This custom implementation appears to be missing support for several JSON schema features like numeric/string constraints (minimum, maxLength), allOf, etc., which could lead to incomplete validation and allow invalid data to pass.

Consider using the validation capabilities of an established library like json_schema_builder (which is already a dependency) to ensure comprehensive and correct validation. If a custom implementation is necessary for performance or other reasons, it would be beneficial to document its limitations.

[gemini-code-assist]

The logic for updating lists within _updateValue handles null values ambiguously. When value is null, current[index] = value sets an element to null rather than removing it, which is inconsistent with how null values are handled for maps (where the key is removed). Additionally, if index == current.length, a null value is added to the list. This behavior could lead to unexpected null entries in lists and deviate from the intended semantics of null meaning 'delete'.

Suggested change

	if (index < current.length) {
	current[index] = value;
	if (value == null) {
	current[index] = value;
	} else {
	current[index] = value;
	}
	} else if (index == current.length) {
	current.add(value);
	} else {
	throw ArgumentError(
	'Index out of bounds for list update: index ($index) is greater '
	'than list length (${current.length}).',
	);
	if (value != null) current.add(value);
	}
	if (index < current.length) {
	if (value == null) {
	current.removeAt(index);
	} else {
	current[index] = value;
	}
	} else if (index == current.length && value != null) {
	current.add(value);
	}

[gemini-code-assist]

The _handleChatMessage method, which listens for UI interaction events, currently only logs the event. In a functional chat application, this should trigger a new request to the LLM to continue the conversation. As it stands, the example is incomplete and could be misleading for developers using it as a reference.

  void _handleChatMessage(ChatMessage event) {
    genUiLogger.info('Received chat message: ${event.toJson()}');
    // For this example, we'll extract the text from the interaction and send it back.
    // A real app might have more complex logic to handle different actions.
    final interactionPart = event.parts.whereType<UiInteractionPart>().firstOrNull;
    if (interactionPart != null) {
      // The interaction part contains a JSON string. For simplicity, we send it as text.
      sendMessage(interactionPart.interaction);
    }
  }

[jacobsimionato]

Now that this does not own an A2UIMessageProcessor etc, it seems like it's adding very little on top of A2uiParserTransformer. Could we remove it and just have A2uiParserTransformer used directly?

[gspencergoog]

Please check the feature/v0.9-migration branch instead.