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.

src/Graph/StellaOps.Graph.Indexer/Ingestion/Policy/PolicyOverlayProcessor.cs

@@ -0,0 +1,85 @@
+using System.Diagnostics;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
+using StellaOps.Graph.Indexer.Ingestion.Sbom;
+
+namespace StellaOps.Graph.Indexer.Ingestion.Policy;
+
+public sealed class PolicyOverlayProcessor
+{
+    private readonly PolicyOverlayTransformer _transformer;
+    private readonly IGraphDocumentWriter _writer;
+    private readonly IPolicyOverlayMetrics _metrics;
+    private readonly ILogger<PolicyOverlayProcessor> _logger;
+
+    public PolicyOverlayProcessor(
+        PolicyOverlayTransformer transformer,
+        IGraphDocumentWriter writer,
+        IPolicyOverlayMetrics metrics,
+        ILogger<PolicyOverlayProcessor> logger)
+    {
+        _transformer = transformer ?? throw new ArgumentNullException(nameof(transformer));
+        _writer = writer ?? throw new ArgumentNullException(nameof(writer));
+        _metrics = metrics ?? throw new ArgumentNullException(nameof(metrics));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    public async Task ProcessAsync(PolicyOverlaySnapshot snapshot, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(snapshot);
+        cancellationToken.ThrowIfCancellationRequested();
+
+        var stopwatch = Stopwatch.StartNew();
+        GraphBuildBatch batch;
+
+        try
+        {
+            batch = _transformer.Transform(snapshot);
+        }
+        catch (Exception ex)
+        {
+            stopwatch.Stop();
+            _metrics.RecordBatch(snapshot.Source, snapshot.Tenant, 0, 0, stopwatch.Elapsed, success: false);
+
+            _logger.LogError(
+                ex,
+                "graph-indexer: failed to transform policy overlay {PolicyPackDigest} for tenant {Tenant}",
+                snapshot.Policy?.PolicyPackDigest ?? string.Empty,
+                snapshot.Tenant);
+
+            throw;
+        }
+
+        try
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            await _writer.WriteAsync(batch, cancellationToken).ConfigureAwait(false);
+            stopwatch.Stop();
+
+            _metrics.RecordBatch(snapshot.Source, snapshot.Tenant, batch.Nodes.Length, batch.Edges.Length, stopwatch.Elapsed, success: true);
+
+            _logger.LogInformation(
+                "graph-indexer: indexed policy overlay {PolicyPackDigest} (effective {EffectiveFrom}) for tenant {Tenant} with {NodeCount} nodes and {EdgeCount} edges in {DurationMs:F2} ms",
+                snapshot.Policy?.PolicyPackDigest ?? string.Empty,
+                (snapshot.Policy?.EffectiveFrom ?? snapshot.CollectedAt).ToUniversalTime(),
+                snapshot.Tenant,
+                batch.Nodes.Length,
+                batch.Edges.Length,
+                stopwatch.Elapsed.TotalMilliseconds);
+        }
+        catch (Exception ex)
+        {
+            stopwatch.Stop();
+            _metrics.RecordBatch(snapshot.Source, snapshot.Tenant, batch.Nodes.Length, batch.Edges.Length, stopwatch.Elapsed, success: false);
+
+            _logger.LogError(
+                ex,
+                "graph-indexer: failed to persist policy overlay {PolicyPackDigest} for tenant {Tenant}",
+                snapshot.Policy?.PolicyPackDigest ?? string.Empty,
+                snapshot.Tenant);
+
+            throw;
+        }
+    }
+}