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/excititor-vex.json

@@ -0,0 +1,34 @@
+{
+  "tenant": "tenant-alpha",
+  "source": "excititor.overlay.v1",
+  "collectedAt": "2025-10-30T12:06:10Z",
+  "eventOffset": 3200,
+  "statement": {
+    "vexSource": "vendor-x",
+    "statementId": "statement-789",
+    "status": "not_affected",
+    "justification": "component not present",
+    "impactStatement": "Library not loaded at runtime",
+    "issuedAt": "2025-10-27T14:30:00Z",
+    "expiresAt": "2026-10-27T14:30:00Z",
+    "contentHash": "sha256:eee555",
+    "provenanceSource": "excititor.vex.v1",
+    "collectedAt": "2025-10-30T12:06:00Z",
+    "eventOffset": 3302
+  },
+  "exemptions": [
+    {
+      "componentPurl": "pkg:nuget/Newtonsoft.Json@13.0.3",
+      "componentSourceType": "inventory",
+      "sbomDigest": "sha256:sbom111",
+      "statementHash": "sha256:eee555",
+      "status": "not_affected",
+      "justification": "component not present",
+      "impactStatement": "Library not loaded at runtime",
+      "evidenceDigest": "sha256:evidence005",
+      "provenanceSource": "excititor.overlay.v1",
+      "collectedAt": "2025-10-30T12:06:10Z",
+      "eventOffset": 3200
+    }
+  ]
+}