Add comprehensive tests for DrawingMessageParser and SarMessageParser

- Implement critical string formatting tests for DrawingMessageParser, ensuring correct message creation, parsing, and validation.
- Validate JSON structure, coordinate formatting, and color index preservation in drawing messages.
- Introduce tests for various drawing types, including LineDrawing and RectangleDrawing, with emphasis on output consistency and error handling.
- Add tests for SarMessageParser, covering message creation, parsing, and format validation, including backward compatibility with old formats.
- Ensure proper handling of special characters, empty notes, and extreme coordinate values in SAR messages.
- Validate that messages conform to the CLAUDE.md specification and maintain compactness and efficiency.

lib/utils/drawing_message_parser.dart

@@ -46,7 +46,7 @@ class DrawingMessageParser {
   /// Sender will be determined from packet metadata on receiving end
   static String createDrawingMessage(MapDrawing drawing) {
     final json = drawing.toNetworkJson();
-    final jsonStr = jsonEncode(json);
+    final jsonStr = jsonEncode(json).toString();
     return '$prefix$jsonStr';
   }