Fix UrlAttribute handling of relative Uris (#115234)

* Update UrlAttribute.cs

* Update UrlAttributeTests.cs

* Fix the test

Author: MihaZupan

src/libraries/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/UrlAttribute.cs

@@ -19,7 +19,7 @@ public override bool IsValid(object? value)
         {
             switch (value)
             {
-                case Uri valueAsUri:
+                case Uri valueAsUri when valueAsUri.IsAbsoluteUri:
                 {
                     return valueAsUri.Scheme == Uri.UriSchemeHttp
                         || valueAsUri.Scheme == Uri.UriSchemeHttps

src/libraries/System.ComponentModel.Annotations/tests/System/ComponentModel/DataAnnotations/UrlAttributeTests.cs

@@ -26,6 +26,8 @@ protected override IEnumerable<TestCase> InvalidValues()
             yield return new TestCase(new UrlAttribute(), new object());
             yield return new TestCase(new UrlAttribute(), new Uri("file:///foo.bar"));
             yield return new TestCase(new UrlAttribute(), new Uri("//foo.png"));
+            yield return new TestCase(new UrlAttribute(), new Uri("/foo.png", UriKind.RelativeOrAbsolute));
+            yield return new TestCase(new UrlAttribute(), new Uri("foo.png", UriKind.Relative));
         }
 
         [Fact]