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/policy-overlay.json

@@ -0,0 +1,31 @@
+{
+  "tenant": "tenant-alpha",
+  "source": "policy.engine.v1",
+  "collectedAt": "2025-10-30T12:07:00Z",
+  "eventOffset": 4200,
+  "policy": {
+    "source": "policy.engine.v1",
+    "policyPackDigest": "sha256:fff666",
+    "policyName": "Default Runtime Policy",
+    "effectiveFrom": "2025-10-28T00:00:00Z",
+    "expiresAt": "2026-01-01T00:00:00Z",
+    "explainHash": "sha256:explain001",
+    "collectedAt": "2025-10-28T00:00:05Z",
+    "eventOffset": 4100
+  },
+  "evaluations": [
+    {
+      "componentPurl": "pkg:nuget/Newtonsoft.Json@13.0.3",
+      "componentSourceType": "inventory",
+      "findingExplainHash": "sha256:explain001",
+      "explainHash": "sha256:explain001",
+      "policyRuleId": "rule:runtime/critical-dependency",
+      "verdict": "fail",
+      "evaluationTimestamp": "2025-10-30T12:07:00Z",
+      "sbomDigest": "sha256:sbom111",
+      "source": "policy.engine.v1",
+      "collectedAt": "2025-10-30T12:07:00Z",
+      "eventOffset": 4200
+    }
+  ]
+}