PAINTROID-761-4: Integrate legacy image parser to app and handle fallback cases

[Amit-Matth]

This Pull Request corresponds to Stage 4 of our backward compatibility integration. It focuses on connecting the legacy model parsing engine to the application's file loader and ensuring robust fallback recovery for corrupted or truncated files.

PAINTROID-761

New Features and Enhancements

• App File Loader Integration: Refactored LoadImageFromFileManager to explicitly support .catrobat and .bin (legacy catrobat-image) file extensions, routing them directly through the CatrobatImage.fromBytes parsing engine.
• Truncated File Safeguard: Added a !reader.hasRemaining safeguard in legacy_model_parser.dart to prevent app crashes when loading .catrobat-image files that were corrupted or cut off early by the legacy Android app. The parser now gracefully stops and recovers whatever strokes were successfully saved before the EOF.
• Fixed Collection Size Decoding: Fixed a byte misalignment issue where Kryo used 1-byte VarInts for generic object sizes, but Paintroid wrote explicit 4-byte Int32s for path actions and colors. Updated the parser to properly consume absolute 4-byte integers for these collections.

Refactorings and Bug Fixes

• Added missing trailing commas in legacy parsing files to match standard Flutter formatting guidelines.
• Added new fallback unit tests in catrobat_image_serializer_test.dart to verify that truncated binary files safely decode without throwing IndexOutOfBound exceptions.

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