Validate email for member models (#17532)

* Validate email for member models

* Add null check or more test cases

* return invalid when not a valid email

* Cleanup

* remove private method in favor of extension

* Remove non used, using statement

---------

Co-authored-by: Elitsa <elm@umbraco.dk>

tests/Umbraco.Tests.UnitTests/Umbraco.Core/ShortStringHelper/StringExtensionsTests.cs

@@ -364,6 +364,16 @@ public class StringExtensionsTests
         TryIsFullPath("   ", false, false); // technically, a valid filename on Linux
     }
 
+    [TestCase("test@test.com", true)]
+    [TestCase("test@test", true)]
+    [TestCase("testtest.com", false)]
+    [TestCase("test@test.dk", true)]
+    [TestCase("test@test.se", true)]
+    [TestCase(null, false)]
+    [TestCase("", false)]
+    [TestCase(" ", false)]
+    public void IsEmail(string? email, bool isEmail) => Assert.AreEqual(isEmail, email.IsEmail());
+
     private static void TryIsFullPath(string path, bool expectedIsFull, bool expectedIsValid = true)
     {
         Assert.AreEqual(expectedIsFull, path.IsFullPath(), "IsFullPath('" + path + "')");