Migrate genui internals to a2ui_core

[andrewkolos]

Context: #811

This is the first of a series of changes RE #811. In this PR, I focussed on trying to keep the diff size down. To this end, this PR preserves the current API surface of package:genui by creating adapters that connect to package:a2ui_core. This diff is still very large, but this approach did manage to cut the diff size to one third 🤷 .

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).
• If my PR is a fork PR, I've checked that [e2e tests] passed.

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:a2ui_core	0.0.1-wip002	ready to publish	a2ui_core-v0.0.1-wip002
package:genai_primitives	0.2.4-wip001	ready to publish	genai_primitives-v0.2.4-wip001
package:genui	0.9.2	(error) pubspec version (0.9.2) and changelog (0.10.0) don't agree
package:genui_a2a	0.9.0	already published at pub.dev
package:json_schema_builder	0.1.4	ready to publish	json_schema_builder-v0.1.4

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[gemini-code-assist]

Code Review

This pull request migrates the runtime substrate of package:genui onto the shared package:a2ui_core implementation, preserving the existing GenUI API as a compatibility facade. Key changes include wrapping core.MessageProcessor in SurfaceController, adapting InMemoryDataModel to wrap a2ui_core.DataModel, and updating BoundValue widgets to use preact_signals internally. Feedback on the changes highlights a critical runtime correctness issue where checking event is! UserActionEvent always evaluates to false due to extension type erasure, suggesting a key-presence check instead. Additionally, it is recommended to reject empty surface IDs across all message types to prevent inefficient buffering.

_{Note: Security Review did not run due to the size of the PR.}

[gemini-code-assist]

At runtime, Dart extension types are completely erased to their representation type (Map<String, Object?>). Therefore, the type check event is! UserActionEvent will always evaluate to false because any UiEvent is represented as a Map at runtime. This causes non-action events to be incorrectly processed as user actions.

To fix this, check for the presence of the 'name' key in the underlying map to distinguish UserActionEvent from other UiEvents. This is a critical correctness issue under the Effective Dart guidelines for extension types.

Suggested change

	void handleUiEvent(UiEvent event) {
	if (event is! UserActionEvent) return;
	void handleUiEvent(UiEvent event) {
	if (!event.toMap().containsKey('name')) return;

[gemini-code-assist]

Instead of only checking for an empty surfaceId on CreateSurfaceMessage, we should reject empty surfaceIds for all message types. Otherwise, messages like UpdateComponentsMessage or UpdateDataModelMessage with an empty surfaceId will be buffered indefinitely under the key "" until they time out, which is inefficient.

We can use the existing _surfaceIdOf helper to check the surfaceId of any incoming message. This aligns with the Effective Dart guidelines for robust error handling and input validation.

  void _handleCoreMessage(core.A2uiMessage coreMessage) {
    final String? surfaceId = _surfaceIdOf(coreMessage);
    if (surfaceId != null && surfaceId.isEmpty) {
      reportError(
        A2uiValidationException(
          'Surface ID cannot be empty',
          surfaceId: '',
          path: 'surfaceId',
        ),
        StackTrace.current,
      );
      return;
    }