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/FileSystemSnapshotFileWriterTests.cs

@@ -0,0 +1,54 @@
+using System.IO;
+using System.Text.Json.Nodes;
+using FluentAssertions;
+using StellaOps.Graph.Indexer.Ingestion.Sbom;
+using Xunit;
+
+namespace StellaOps.Graph.Indexer.Tests;
+
+public sealed class FileSystemSnapshotFileWriterTests : IDisposable
+{
+    private readonly string _root = Path.Combine(Path.GetTempPath(), $"graph-snapshots-{Guid.NewGuid():N}");
+
+    [Fact]
+    public async Task WriteJsonAsync_writes_canonical_json()
+    {
+        var writer = new FileSystemSnapshotFileWriter(_root);
+        var json = new JsonObject
+        {
+            ["b"] = "value2",
+            ["a"] = "value1"
+        };
+
+        await writer.WriteJsonAsync("manifest.json", json, CancellationToken.None);
+
+        var content = await File.ReadAllTextAsync(Path.Combine(_root, "manifest.json"));
+        content.Should().Be("{\"a\":\"value1\",\"b\":\"value2\"}");
+    }
+
+    [Fact]
+    public async Task WriteJsonLinesAsync_writes_each_object_on_new_line()
+    {
+        var writer = new FileSystemSnapshotFileWriter(_root);
+        var items = new[]
+        {
+            new JsonObject { ["id"] = "1", ["kind"] = "component" },
+            new JsonObject { ["id"] = "2", ["kind"] = "artifact" }
+        };
+
+        await writer.WriteJsonLinesAsync("nodes.jsonl", items, CancellationToken.None);
+
+        var lines = await File.ReadAllLinesAsync(Path.Combine(_root, "nodes.jsonl"));
+        lines.Should().HaveCount(2);
+        lines[0].Should().Be("{\"id\":\"1\",\"kind\":\"component\"}");
+        lines[1].Should().Be("{\"id\":\"2\",\"kind\":\"artifact\"}");
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_root))
+        {
+            Directory.Delete(_root, recursive: true);
+        }
+    }
+}