Fix Nuget Test Failures (#500)

Refactor Nuget Project Manager Tests to address test failures.
Fix a potential null reference exception in Nuget Project Manager.

Author: gfs

src/Shared/PackageManagers/NuGetProjectManager.cs

@@ -280,7 +280,7 @@ private async Task UpdateVersionMetadata(PackageMetadata metadata, NuGetPackageV
             }
 
             // Dependencies
-            IList<PackageDependencyGroup> dependencyGroups = packageVersionMetadata.DependencySets.ToList();
+            IList<PackageDependencyGroup> dependencyGroups = packageVersionMetadata.DependencySets?.ToList() ?? new List<PackageDependencyGroup>();
             metadata.Dependencies ??= dependencyGroups.SelectMany(group => group.Packages, (dependencyGroup, package) => new { dependencyGroup, package})
                 .Select(dependencyGroupAndPackage => new Dependency() { Package = dependencyGroupAndPackage.package.ToString(), Framework = dependencyGroupAndPackage.dependencyGroup.TargetFramework?.ToString()})
                 .ToList();

src/oss-tests/Helpers/NuGetTestHelper.cs

@@ -0,0 +1,110 @@
+// Copyright (c) Microsoft Corporation. Licensed under the MIT License.
+
+namespace Microsoft.CST.OpenSource.Tests.Helpers;
+
+using Contracts;
+using Model.Metadata;
+using Moq;
+using NuGet.Packaging;
+using PackageUrl;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+
+/// <summary>
+/// Helper class for working with NuGet metadata in tests
+/// </summary>
+public static class NuGetTestHelper
+{
+    /// <summary>
+    /// Creates a NuGetPackageVersionMetadata object for use in tests with 
+    /// proper initialization of all required properties.
+    /// </summary>
+    public static NuGetPackageVersionMetadata CreateMetadata(
+        string name, 
+        string version, 
+        string description = "", 
+        string authors = "", 
+        Uri? catalogUri = null,
+        string tags = "")
+    {
+        // Instead of directly creating the metadata object with the problematic IEnumerable property,
+        // we'll create a TestNuGetPackageVersionMetadata that uses a List instead
+        return new TestNuGetPackageVersionMetadata
+        {
+            Name = name,
+            Version = version,
+            Description = description,
+            Authors = authors,
+            Tags = tags,
+            CatalogUri = catalogUri ?? new Uri($"https://api.nuget.org/v3/catalog0/data/2022.03.11.23.17.27/{name}.{version}.json"),
+            DependencySets = new List<PackageDependencyGroup>() // Empty list to avoid serialization issues
+        };
+    }
+
+    /// <summary>
+    /// Sets up a mock package actions object for testing NuGet package operations
+    /// </summary>
+    /// <returns>A mocked IManagerPackageActions that returns the specified metadata and versions</returns>
+    public static IManagerPackageActions<NuGetPackageVersionMetadata> SetupPackageActions(
+        PackageURL purl,
+        NuGetPackageVersionMetadata metadata,
+        IEnumerable<string>? versions = null)
+    {
+        Mock<IManagerPackageActions<NuGetPackageVersionMetadata>> mockActions = new();
+        
+        // Set up GetMetadataAsync to return our metadata
+        mockActions.Setup(m => m.GetMetadataAsync(
+                It.Is<PackageURL>(p => p.Name.Equals(purl.Name, StringComparison.OrdinalIgnoreCase) && 
+                                       (purl.Version == null || p.Version == purl.Version)),
+                It.IsAny<bool>(),
+                It.IsAny<CancellationToken>()))
+            .ReturnsAsync(metadata);
+
+        // If versions are provided, set up GetAllVersionsAsync and GetLatestVersionAsync
+        if (versions != null)
+        {
+            string[] versionArray = versions as string[] ?? versions.ToArray();
+            
+            mockActions.Setup(m => m.GetAllVersionsAsync(
+                    It.Is<PackageURL>(p => p.Name.Equals(purl.Name, StringComparison.OrdinalIgnoreCase)),
+                    It.IsAny<bool>(),
+                    It.IsAny<bool>(),
+                    It.IsAny<CancellationToken>()))
+                .ReturnsAsync(versionArray);
+                
+            if (versionArray.Length > 0)
+            {
+                mockActions.Setup(m => m.GetLatestVersionAsync(
+                        It.Is<PackageURL>(p => p.Name.Equals(purl.Name, StringComparison.OrdinalIgnoreCase)),
+                        It.IsAny<bool>(),
+                        It.IsAny<bool>(),
+                        It.IsAny<CancellationToken>()))
+                    .ReturnsAsync(versionArray[0]);
+            }
+        }
+        
+        // Set up DoesPackageExistAsync to return true for this package
+        mockActions.Setup(m => m.DoesPackageExistAsync(
+                It.Is<PackageURL>(p => p.Name.Equals(purl.Name, StringComparison.OrdinalIgnoreCase)),
+                It.IsAny<bool>(),
+                It.IsAny<CancellationToken>()))
+            .ReturnsAsync(true);
+
+        return mockActions.Object;
+    }
+}
+
+/// <summary>
+/// A test-specific implementation of NuGetPackageVersionMetadata that uses a List for DependencySets
+/// to avoid serialization issues
+/// </summary>
+public record TestNuGetPackageVersionMetadata : NuGetPackageVersionMetadata
+{
+    // Override the problematic property with a concrete List implementation
+    public new List<PackageDependencyGroup> DependencySets { get; set; } = new();
+    
+    // This property is accessed in some places so we need to ensure it returns our concrete List
+    public IEnumerable<PackageDependencyGroup> DependencySetsInternal => DependencySets;
+}