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/Findings/StellaOps.Findings.Ledger/Domain/LedgerEventModels.cs

@@ -0,0 +1,85 @@
+using System.Text.Json.Nodes;
+
+namespace StellaOps.Findings.Ledger.Domain;
+
+public sealed record LedgerEventDraft(
+    string TenantId,
+    Guid ChainId,
+    long SequenceNumber,
+    Guid EventId,
+    string EventType,
+    string PolicyVersion,
+    string FindingId,
+    string ArtifactId,
+    Guid? SourceRunId,
+    string ActorId,
+    string ActorType,
+    DateTimeOffset OccurredAt,
+    DateTimeOffset RecordedAt,
+    JsonObject Payload,
+    JsonObject CanonicalEnvelope,
+    string? ProvidedPreviousHash);
+
+public sealed record LedgerEventRecord(
+    string TenantId,
+    Guid ChainId,
+    long SequenceNumber,
+    Guid EventId,
+    string EventType,
+    string PolicyVersion,
+    string FindingId,
+    string ArtifactId,
+    Guid? SourceRunId,
+    string ActorId,
+    string ActorType,
+    DateTimeOffset OccurredAt,
+    DateTimeOffset RecordedAt,
+    JsonObject EventBody,
+    string EventHash,
+    string PreviousHash,
+    string MerkleLeafHash,
+    string CanonicalJson);
+
+public sealed record LedgerChainHead(
+    long SequenceNumber,
+    string EventHash,
+    DateTimeOffset RecordedAt);
+
+public enum LedgerWriteStatus
+{
+    Success,
+    Idempotent,
+    ValidationFailed,
+    Conflict
+}
+
+public sealed record LedgerWriteResult(
+    LedgerWriteStatus Status,
+    LedgerEventRecord? Record,
+    IReadOnlyList<string> Errors,
+    LedgerEventRecord? ExistingRecord,
+    string? ConflictCode)
+{
+    public static LedgerWriteResult ValidationFailed(params string[] errors)
+        => new(LedgerWriteStatus.ValidationFailed, null, errors, null, null);
+
+    public static LedgerWriteResult Conflict(string code, params string[] errors)
+        => new(LedgerWriteStatus.Conflict, null, errors, null, code);
+
+    public static LedgerWriteResult Idempotent(LedgerEventRecord record)
+        => new(LedgerWriteStatus.Idempotent, record, Array.Empty<string>(), record, null);
+
+    public static LedgerWriteResult Success(LedgerEventRecord record)
+        => new(LedgerWriteStatus.Success, record, Array.Empty<string>(), null, null);
+}
+
+public sealed class LedgerDuplicateEventException : Exception
+{
+    public LedgerDuplicateEventException(Guid eventId, Exception innerException)
+        : base($"Ledger event {eventId} already exists.", innerException)
+    {
+        EventId = eventId;
+    }
+
+    public Guid EventId { get; }
+}