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.

src/Concelier/__Libraries/StellaOps.Concelier.Connector.Common/Fetch/SourceFetchService.cs

@@ -1,3 +1,4 @@
+using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Globalization;
@@ -298,10 +299,44 @@ public sealed class SourceFetchService
         }
         catch (JsonException ex)
         {
-            throw new InvalidOperationException($"Raw advisory payload from {request.SourceName} is not valid JSON ({request.RequestUri}).", ex);
+            var fallbackDocument = CreateFallbackContentDocument(request, contentBytes, ex);
+            return fallbackDocument;
         }
     }
 
+    private static JsonDocument CreateFallbackContentDocument(
+        SourceFetchRequest request,
+        byte[] contentBytes,
+        JsonException parseException)
+    {
+        var payload = new Dictionary<string, object?>
+        {
+            ["type"] = "non-json",
+            ["encoding"] = "base64",
+            ["source"] = request.SourceName,
+            ["uri"] = request.RequestUri.ToString(),
+            ["mediaTypeHint"] = request.AcceptHeaders?.FirstOrDefault(),
+            ["parseError"] = parseException.Message,
+            ["raw"] = Convert.ToBase64String(contentBytes),
+        };
+
+        try
+        {
+            var text = Encoding.UTF8.GetString(contentBytes);
+            if (!string.IsNullOrWhiteSpace(text))
+            {
+                payload["text"] = text;
+            }
+        }
+        catch
+        {
+            // ignore decoding failures; base64 field already present
+        }
+
+        var buffer = JsonSerializer.SerializeToUtf8Bytes(payload, new JsonSerializerOptions(JsonSerializerDefaults.Web));
+        return JsonDocument.Parse(buffer);
+    }
+
     private static ImmutableDictionary<string, string> BuildProvenance(
         SourceFetchRequest request,
         HttpResponseMessage response,