Added tests (WIP)

lib/src/extensions/time_only_extensions.dart

@@ -31,8 +31,16 @@ extension TimeOnlyExtensions on TimeOnly {
     );
   }
 
-  /// Converts the [TimeOnly] to a string in the format `HH:mm:ss.SSS`.
+  /// Converts the [TimeOnly] to a string in the format `HH:mm:ss` or
+  /// `HH:mm:ss.SSS` if milliseconds are present.
   String toRfc3339String() {
-    return '${hours.toString().padLeft(2, '0')}:${minutes.toString().padLeft(2, '0')}:${seconds.toString().padLeft(2, '0')}.${milliseconds.toString().padLeft(3, '0')}';
+    final String fractionString;
+    if (milliseconds > 0) {
+      fractionString = '.${milliseconds.toString().padLeft(3, '0')}';
+    } else {
+      fractionString = '';
+    }
+
+    return '${hours.toString().padLeft(2, '0')}:${minutes.toString().padLeft(2, '0')}:${seconds.toString().padLeft(2, '0')}$fractionString';
   }
 }

lib/src/time_only.dart

@@ -19,7 +19,7 @@ abstract class TimeOnly {
   /// This factory uses the [DateTime.parse] method to create a new
   /// [_TimeOnlyImpl] instance from a string.
   factory TimeOnly.fromDateTimeString(String dateTimeString) {
-    final dateTime = DateTime.parse(dateTimeString);
+    final dateTime = DateTime.parse('2024-01-01 $dateTimeString');
 
     return TimeOnly.fromDateTime(dateTime);
   }

test/time_only_test.dart

@@ -0,0 +1,35 @@
+import 'package:kiota_abstractions/kiota_abstractions.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('TimeOnly', () {
+    test('fromDateTimeString and toRfc3339String', () {
+      expect(
+        TimeOnly.fromDateTimeString('12:34:56').toRfc3339String(),
+        '12:34:56',
+      );
+      expect(
+        TimeOnly.fromDateTimeString('12:34').toRfc3339String(),
+        '12:34:00',
+      );
+      expect(TimeOnly.fromDateTimeString('12').toRfc3339String(), '12:00:00');
+    });
+
+    test('fromDateTime', () {
+      expect(
+        TimeOnly.fromDateTime(DateTime(2024, 1, 1, 12, 34, 56))
+            .toRfc3339String(),
+        '12:34:56',
+      );
+    });
+
+    test('fromComponents', () {
+      expect(
+        TimeOnly.fromComponents(12, 34, 56, 789).toRfc3339String(),
+        '12:34:56.789',
+      );
+      expect(TimeOnly.fromComponents(12, 34, 56).toRfc3339String(), '12:34:56');
+      expect(TimeOnly.fromComponents(12, 34).toRfc3339String(), '12:34:00');
+    });
+  });
+}