Add tests for filtering validation

Author: MaggieKimani1

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

@@ -0,0 +1,49 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+using System;
+using Microsoft.OpenApi.Models;
+using Microsoft.OpenApi.Services;
+using OpenAPIService.Test;
+using Xunit;
+
+namespace Microsoft.OpenApi.Tests.Services
+{
+    public class OpenApiFilterServiceTests
+    {
+        private const string Title = "Partial Graph API";
+        private const string GraphVersion = "mock";
+        private readonly OpenApiFilterService _openApiFilterService;
+        private readonly OpenApiDocument _openApiDocumentMock;
+
+        public OpenApiFilterServiceTests()
+        {
+            _openApiFilterService = new OpenApiFilterService();
+            _openApiDocumentMock = OpenApiDocumentMock.CreateOpenApiDocument();
+        }
+
+        [Theory]
+        [InlineData("users.user.ListUser")]
+        [InlineData("users.user.GetUser")]
+        [InlineData("administrativeUnits.restore")]
+        [InlineData("graphService.GetGraphService")]
+        public void ReturnFilteredOpenApiDocumentBasedOnOperationIds(string operationId)
+        {
+            // Act
+            var predicate = _openApiFilterService.CreatePredicate(operationId);
+            var subsetOpenApiDocument = _openApiFilterService.CreateFilteredDocument(_openApiDocumentMock, Title, GraphVersion, predicate);
+
+            // Assert
+            Assert.NotNull(subsetOpenApiDocument);
+            Assert.Single(subsetOpenApiDocument.Paths);
+        }
+
+        [Fact]
+        public void ThrowsInvalidOperationExceptionInCreatePredicateWhenInvalidOperationIdIsSpecified()
+        {
+            // Act and Assert
+            var message = Assert.Throws<InvalidOperationException>(() =>_openApiFilterService.CreatePredicate(null)).Message;
+            Assert.Equal("OperationId needs to be specified.", message);
+        }
+    }
+}