test: text style tests

frontend/app_flowy/packages/appflowy_editor/test/extensions/text_style_extension_test.dart

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+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:appflowy_editor/src/extensions/text_style_extension.dart';
+
+void main() {
+  group('TextStyleExtensions::', () {
+    const style = TextStyle(
+      color: Colors.blue,
+      backgroundColor: Colors.white,
+      fontSize: 14,
+      height: 100,
+      wordSpacing: 2,
+      fontWeight: FontWeight.w700,
+    );
+
+    const otherStyle = TextStyle(
+      color: Colors.red,
+      backgroundColor: Colors.black,
+      fontSize: 12,
+      height: 10,
+      wordSpacing: 1,
+    );
+    test('combine', () {
+      final result = style.combine(otherStyle);
+      expect(result.color, Colors.red);
+      expect(result.backgroundColor, Colors.black);
+      expect(result.fontSize, 12);
+      expect(result.height, 10);
+      expect(result.wordSpacing, 1);
+    });
+
+    test('combine - return this', () {
+      final result = style.combine(null);
+      expect(result, style);
+    });
+
+    test('combine - return null with inherit', () {
+      final styleCopy = otherStyle.copyWith(inherit: false);
+      final result = style.combine(styleCopy);
+      expect(result, styleCopy);
+    });
+  });
+}