Add validation check and add test

Author: MaggieKimani1

src/Microsoft.OpenApi/Services/OpenApiFilterService.cs

@@ -23,6 +23,10 @@ public static class OpenApiFilterService
         public static Func<OpenApiOperation, bool> CreatePredicate(string operationIds = null, string tags = null)
         {
             Func<OpenApiOperation, bool> predicate;
+            if (!string.IsNullOrEmpty(operationIds) && !string.IsNullOrEmpty(tags))
+            {
+                throw new InvalidOperationException("Cannot specify both operationIds and tags at the same time.");
+            }
             if (operationIds != null)
             {
                 if (operationIds == "*")
@@ -49,7 +53,6 @@ public static Func<OpenApiOperation, bool> CreatePredicate(string operationIds =
                     predicate = (o) => o.Tags.Any(t => tagsArray.Contains(t.Name));
                 }
             }
-
             else
             {
                 throw new InvalidOperationException("Either operationId(s) or tag(s) need to be specified.");

test/Microsoft.OpenApi.Tests/Services/OpenApiFilterServiceTests.cs

@@ -46,8 +46,11 @@ public void ReturnFilteredOpenApiDocumentBasedOnOperationIdsAndTags(string opera
         public void ThrowsInvalidOperationExceptionInCreatePredicateWhenInvalidArgumentsArePassed()
         {
             // Act and Assert
-            var message = Assert.Throws<InvalidOperationException>(() =>OpenApiFilterService.CreatePredicate(null, null)).Message;
-            Assert.Equal("Either operationId(s) or tag(s) need to be specified.", message);
+            var message1 = Assert.Throws<InvalidOperationException>(() => OpenApiFilterService.CreatePredicate(null, null)).Message;
+            Assert.Equal("Either operationId(s) or tag(s) need to be specified.", message1);
+
+            var message2 = Assert.Throws<InvalidOperationException>(() => OpenApiFilterService.CreatePredicate("users.user.ListUser", "users.user")).Message;
+            Assert.Equal("Cannot specify both operationIds and tags at the same time.", message2);
         }
     }
 }