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/README.md

@@ -0,0 +1,14 @@
+# StellaOps Graph Indexer Tests
+
+The Graph Indexer integration tests exercise the Mongo-backed document writer.  
+To run the suite locally (or in CI) you **must** point the tests at a reachable MongoDB instance.
+
+## Required environment
+
+```bash
+export STELLAOPS_TEST_MONGO_URI="mongodb://user:pass@host:27017/test-db"
+```
+
+The harness will try the connection string above first, then fall back to `mongodb://127.0.0.1:27017`, and finally to an embedded MongoDB instance via Mongo2Go. If neither the URI nor a local `mongod` is reachable, the Mongo writer tests are skipped with a diagnostic message.
+
+CI pipelines are configured to fail early when `STELLAOPS_TEST_MONGO_URI` is missing so that the integration coverage always runs with a known database.