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/concelier-linkset.json

@@ -0,0 +1,32 @@
+{
+  "tenant": "tenant-alpha",
+  "source": "concelier.overlay.v1",
+  "linksetDigest": "sha256:linkset001",
+  "collectedAt": "2025-10-30T12:05:10Z",
+  "eventOffset": 3100,
+  "advisory": {
+    "source": "concelier.linkset.v1",
+    "advisorySource": "ghsa",
+    "advisoryId": "GHSA-1234-5678-90AB",
+    "severity": "HIGH",
+    "publishedAt": "2025-10-25T09:00:00Z",
+    "contentHash": "sha256:ddd444",
+    "linksetDigest": "sha256:linkset001"
+  },
+  "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,
+      "source": "concelier.overlay.v1",
+      "collectedAt": "2025-10-30T12:05:10Z",
+      "eventOffset": 3100
+    }
+  ]
+}