Refactor code structure for improved readability and maintainability; optimize performance in key functions.

src/Graph/__Tests/StellaOps.Graph.Api.Tests/LineageServiceTests.cs

@@ -0,0 +1,31 @@
+using System.Threading;
+using System.Threading.Tasks;
+using StellaOps.Graph.Api.Contracts;
+using StellaOps.Graph.Api.Services;
+using Xunit;
+
+namespace StellaOps.Graph.Api.Tests;
+
+public sealed class LineageServiceTests
+{
+    [Fact]
+    public async Task GetLineageAsync_ReturnsSbomAndArtifactChain()
+    {
+        var repository = new InMemoryGraphRepository();
+        var service = new InMemoryGraphLineageService(repository);
+
+        var request = new GraphLineageRequest
+        {
+            SbomDigest = "sha256:sbom-b",
+            MaxDepth = 3
+        };
+
+        var response = await service.GetLineageAsync("acme", request, CancellationToken.None);
+
+        Assert.Contains(response.Nodes, node => node.Id == "gn:acme:sbom:sha256:sbom-b");
+        Assert.Contains(response.Nodes, node => node.Id == "gn:acme:sbom:sha256:sbom-a");
+        Assert.Contains(response.Nodes, node => node.Id == "gn:acme:artifact:sha256:abc");
+        Assert.Contains(response.Edges, edge => edge.Kind == "SBOM_LINEAGE_PARENT");
+        Assert.Contains(response.Edges, edge => edge.Kind == "SBOM_VERSION_OF");
+    }
+}