refactor: migrate to json_serializable-based toJson and mixin toString

[inuEbisu]

Current Progress

Implementing json_serializable + JsonToStringMixin for core/ctap2 and src/server entities and requests:

• Replace handwritten toString with JsonToStringMixin
• Generate toJson via json_serializable (createFactory: false, no fromJson needed)
• No behavioral changes; purely for logging/serialization consistency
• Configure github workflow CI for dart run build_runner build check.

Special Cases

Two classes remain with manual toJson implementation:

• CoseKey: extends MapView<int, dynamic>, json_serializable incompatible. Manual toJson handles numeric key conversion.
• CtapError: extends Error, inherited StackTrace causes generation issues. Simple manual toJson for status + name.

[Copilot]

Pull Request Overview

This PR refactors the codebase to use json_serializable for automatic toJson generation and introduces a JsonToStringMixin for consistent toString behavior across entities and requests. It replaces manual toString implementations with JSON-based serialization while maintaining the same logging/debugging functionality.

• Migrates 20+ classes to use @JsonSerializable annotations with automatic toJson generation
• Introduces JsonToStringMixin that provides toString() via jsonEncode(toJson())
• Adds CI workflow validation to ensure generated code stays in sync

Reviewed Changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pubspec.yaml	Adds json_annotation, json_serializable, and build_runner dependencies
lib/src/utils/serialization.dart	Introduces JsonToStringMixin for consistent JSON-based toString
Generated .g.dart files	Auto-generated toJson factory functions for annotated classes
CTAP2 entities and requests	Converts manual toString to JsonSerializable + JsonToStringMixin
Server entities	Migrates server-side data classes to use json_serializable
lib/src/cose.dart	Updates CoseKey with custom toJson due to MapView inheritance
lib/src/ctap.dart	Converts CtapResponse and CtapError to use new serialization pattern
.github/workflows/test.yml	Adds build_runner validation to CI
example/json_serializable_example.dart	Demonstrates new serialization capabilities

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