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.