TF-3088 Fix the ending date of custom date range isn't included in the search

test/features/base/mixin/date_range_picker_mixin_test.dart

@@ -0,0 +1,56 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tmail_ui_user/features/base/mixin/date_range_picker_mixin.dart';
+
+class DateRangePickerMixinTest with DateRangePickerMixin {}
+
+void main() {
+  final dateRangePickerMixinTest = DateRangePickerMixinTest();
+
+  group('DateRangePickerMixin::validateDateRange::', () {
+    test('should set startDate to midnight and endDate to 23:59', () {
+      // Arrange
+      final DateTime startDate = DateTime(2024, 8, 30, 12, 30);
+      final DateTime endDate = DateTime(2024, 8, 31, 15, 45);
+
+      // Act
+      final result = dateRangePickerMixinTest.validateDateRange(startDate: startDate, endDate: endDate);
+
+      // Assert
+      expect(result.startDate, DateTime(2024, 8, 30, 0, 0));
+      expect(result.endDate, DateTime(2024, 8, 31, 23, 59));
+    });
+
+    test('should return null startDate and endDate if null input', () {
+      // Act
+      final result = dateRangePickerMixinTest.validateDateRange(startDate: null, endDate: null);
+
+      // Assert
+      expect(result.startDate, isNull);
+      expect(result.endDate, isNull);
+    });
+
+    test('should set startDate to midnight and keep endDate null if only startDate is provided', () {
+      // Arrange
+      final DateTime startDate = DateTime(2024, 8, 30, 12, 30);
+
+      // Act
+      final result = dateRangePickerMixinTest.validateDateRange(startDate: startDate, endDate: null);
+
+      // Assert
+      expect(result.startDate, DateTime(2024, 8, 30, 0, 0));
+      expect(result.endDate, isNull);
+    });
+
+    test('should set endDate to 23:59 and keep startDate null if only endDate is provided', () {
+      // Arrange
+      final DateTime endDate = DateTime(2024, 8, 31, 15, 45);
+
+      // Act
+      final result = dateRangePickerMixinTest.validateDateRange(startDate: null, endDate: endDate);
+
+      // Assert
+      expect(result.startDate, isNull);
+      expect(result.endDate, DateTime(2024, 8, 31, 23, 59));
+    });
+  });
+}