Add unit tests for SBOM ingestion and transformation

- Implement `SbomIngestServiceCollectionExtensionsTests` to verify the SBOM ingestion pipeline exports snapshots correctly.
- Create `SbomIngestTransformerTests` to ensure the transformation produces expected nodes and edges, including deduplication of license nodes and normalization of timestamps.
- Add `SbomSnapshotExporterTests` to test the export functionality for manifest, adjacency, nodes, and edges.
- Introduce `VexOverlayTransformerTests` to validate the transformation of VEX nodes and edges.
- Set up project file for the test project with necessary dependencies and configurations.
- Include JSON fixture files for testing purposes.

tests/Graph/StellaOps.Graph.Indexer.Tests/PolicyOverlayTransformerTests.cs

@@ -0,0 +1,107 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text.Json;
+using System.Text.Json.Nodes;
+using FluentAssertions;
+using StellaOps.Graph.Indexer.Ingestion.Policy;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace StellaOps.Graph.Indexer.Tests;
+
+public sealed class PolicyOverlayTransformerTests
+{
+    private readonly ITestOutputHelper _output;
+
+    public PolicyOverlayTransformerTests(ITestOutputHelper output)
+    {
+        _output = output;
+    }
+
+    private static readonly string FixturesRoot =
+        Path.Combine(AppContext.BaseDirectory, "Fixtures", "v1");
+
+    private static readonly HashSet<string> ExpectedNodeKinds = new(StringComparer.Ordinal)
+    {
+        "policy_version"
+    };
+
+    private static readonly HashSet<string> ExpectedEdgeKinds = new(StringComparer.Ordinal)
+    {
+        "GOVERNS_WITH"
+    };
+
+    [Fact]
+    public void Transform_projects_policy_nodes_and_governs_with_edges()
+    {
+        var snapshot = LoadSnapshot("policy-overlay.json");
+        var transformer = new PolicyOverlayTransformer();
+
+        var batch = transformer.Transform(snapshot);
+
+        var expectedNodes = LoadArray("nodes.json")
+            .Cast<JsonObject>()
+            .Where(node => ExpectedNodeKinds.Contains(node["kind"]!.GetValue<string>()))
+            .OrderBy(node => node["id"]!.GetValue<string>(), StringComparer.Ordinal)
+            .ToArray();
+
+        var expectedEdges = LoadArray("edges.json")
+            .Cast<JsonObject>()
+            .Where(edge => ExpectedEdgeKinds.Contains(edge["kind"]!.GetValue<string>()))
+            .OrderBy(edge => edge["id"]!.GetValue<string>(), StringComparer.Ordinal)
+            .ToArray();
+
+        var actualNodes = batch.Nodes
+            .Where(node => ExpectedNodeKinds.Contains(node["kind"]!.GetValue<string>()))
+            .OrderBy(node => node["id"]!.GetValue<string>(), StringComparer.Ordinal)
+            .ToArray();
+
+        var actualEdges = batch.Edges
+            .Where(edge => ExpectedEdgeKinds.Contains(edge["kind"]!.GetValue<string>()))
+            .OrderBy(edge => edge["id"]!.GetValue<string>(), StringComparer.Ordinal)
+            .ToArray();
+
+        actualNodes.Length.Should().Be(expectedNodes.Length);
+        actualEdges.Length.Should().Be(expectedEdges.Length);
+
+        for (var i = 0; i < expectedNodes.Length; i++)
+        {
+            if (!JsonNode.DeepEquals(expectedNodes[i], actualNodes[i]))
+            {
+                _output.WriteLine($"Expected Node: {expectedNodes[i]}");
+                _output.WriteLine($"Actual Node:   {actualNodes[i]}");
+            }
+
+            JsonNode.DeepEquals(expectedNodes[i], actualNodes[i]).Should().BeTrue();
+        }
+
+        for (var i = 0; i < expectedEdges.Length; i++)
+        {
+            if (!JsonNode.DeepEquals(expectedEdges[i], actualEdges[i]))
+            {
+                _output.WriteLine($"Expected Edge: {expectedEdges[i]}");
+                _output.WriteLine($"Actual Edge:   {actualEdges[i]}");
+            }
+
+            JsonNode.DeepEquals(expectedEdges[i], actualEdges[i]).Should().BeTrue();
+        }
+    }
+
+    private static PolicyOverlaySnapshot LoadSnapshot(string fileName)
+    {
+        var path = Path.Combine(FixturesRoot, fileName);
+        var json = File.ReadAllText(path);
+        return JsonSerializer.Deserialize<PolicyOverlaySnapshot>(json, new JsonSerializerOptions
+        {
+            PropertyNameCaseInsensitive = true
+        })!;
+    }
+
+    private static JsonArray LoadArray(string fileName)
+    {
+        var path = Path.Combine(FixturesRoot, fileName);
+        return (JsonArray)JsonNode.Parse(File.ReadAllText(path))!;
+    }
+}