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/Fixtures/v1/linkset-snapshot.json

@@ -0,0 +1,29 @@
+{
+  tenant: tenant-alpha,
+  source: concelier.overlay.v1,
+  linksetDigest: sha256:linkset001,
+  collectedAt: 2025-10-30T12:05:00Z,
+  eventOffset: 2201,
+  advisory: {
+    source: concelier.linkset.v1,
+    advisorySource: ghsa,
+    advisoryId: GHSA-1234-5678-90AB,
+    contentHash: sha256:ddd444,
+    severity: HIGH,
+    publishedAt: 2025-10-25T09:00:00Z
+  },
+  components: [
+    {
+      purl: pkg:nuget/Newtonsoft.Json@13.0.3,
+      sourceType: inventory,
+      sbomDigest: sha256:sbom111,
+      evidenceDigest: sha256:evidence004,
+      matchedVersions: [13.0.3],
+      cvss: 8.1,
+      confidence: 0.9,
+      collectedAt: 2025-10-30T12:05:10Z,
+      eventOffset: 3100,
+      source: concelier.overlay.v1
+    }
+  ]
+}