PAINTROID-729 Refactor Converters

[Amit-Matth]

This pull request refactors the path serialization logic to eliminate external converters and centralize JSON logic within the models themselves.

PAINTROID-729

New Features and Enhancements

• Unified Path Serialization: Introduced a new PathModel that encapsulates both the native Flutter Path and its drawing action history.
• Self-Serializing Models: Moved fromJson and toJson logic directly into the PathModel and PathAction classes using json_serializable.
• Version Awareness: Implemented versioning for PathModel (consistent with CatrobatImage) to ensure robust project loading and future-proofing.

Refactorings and Bug Fixes

• Converter Removal: Deleted the obsolete PathActionConverter and PathWithActionHistoryConverter classes.
• Improved Maintainability: Simplified PathCommand and LineCommand by using explicitToJson: true, allowing for cleaner nested object serialization.
• Backward Compatibility: Added a safety check in PathModel.fromJson to correctly handle and migrate legacy projects saved without a version number.
• Test Suite Modernization: Updated over 40 unit and widget tests to align with the new model architecture.
• Tool Refactoring: Updated BrushTool and LineTool to utilize the improved PathModel lifecycle.

Checklist

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