feat(a2ui_message): make toJson compatible with fromJson

[matiasleyba]

Description

• Each A2uiMessage subclass’s toJson() now returns the wrapped envelope (version + one of createSurface / updateComponents / updateDataModel / deleteSurface) instead of a flat payload.
• This matches the shape that A2uiMessage.fromJson() expects, so round-trip works.

Why

• Use case: Persisting messages locally (e.g. saving conversation history) and reloading with A2uiMessage.fromJson(saved).
• Previously, fromJson(message.toJson()) failed because the root map had no action key.
• Before: To save a message you had to switch on each type (CreateSurface, UpdateComponents, etc.) and build JSON per variant.
  • Now: You can call message.toJson() on any A2uiMessage and save without type checks.

---

• CreateSurface, UpdateComponents, UpdateDataModel, and DeleteSurface now put their payload under the corresponding key.

• Round-trip tests were added

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 ///).

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[gemini-code-assist]

Code Review

This pull request refactors the toJson() methods for A2uiMessage subclasses to make them compatible with A2uiMessage.fromJson(), enabling round-trip serialization. The changes are well-implemented and include corresponding updates to tests and new round-trip tests. My feedback focuses on improving maintainability by using constants for repeated string literals like the API version and JSON keys, and a minor cleanup in the new test code.

[gemini-code-assist]

The version string 'v0.9' is repeated across all toJson implementations and is also checked in A2uiMessage.fromJson. To improve maintainability and prevent potential inconsistencies when updating the version, consider defining this as a constant. For example, you could add a private constant at the top of this file: const String _kApiVersion = 'v0.9'; and use it in all relevant places.

[gemini-code-assist]

There are several string literals used as JSON keys here and in the corresponding fromJson factory (e.g., 'createSurface', 'catalogId', 'theme', 'sendDataModel'). To prevent typos and improve maintainability, it's a good practice to define these as constants, similar to how surfaceIdKey is used. This would make the code safer and easier to refactor.