feat: add Attestation Chain and Triage Evidence API clients and models

- Implemented Attestation Chain API client with methods for verifying, fetching, and managing attestation chains.
- Created models for Attestation Chain, including DSSE envelope structures and verification results.
- Developed Triage Evidence API client for fetching finding evidence, including methods for evidence retrieval by CVE and component.
- Added models for Triage Evidence, encapsulating evidence responses, entry points, boundary proofs, and VEX evidence.
- Introduced mock implementations for both API clients to facilitate testing and development.

src/Signals/__Tests/StellaOps.Signals.Tests/CallgraphIngestionServiceTests.cs

@@ -33,12 +33,14 @@ public class CallgraphIngestionServiceTests
         var resolver = new StubParserResolver(parser);
         var options = Microsoft.Extensions.Options.Options.Create(new SignalsOptions());
         var reachabilityStore = new InMemoryReachabilityStoreRepository(_timeProvider);
+        var callGraphSyncService = new StubCallGraphSyncService();
         var service = new CallgraphIngestionService(
             resolver,
             _artifactStore,
             _repository,
             reachabilityStore,
             _normalizer,
+            callGraphSyncService,
             options,
             _timeProvider,
             NullLogger<CallgraphIngestionService>.Instance);
@@ -189,4 +191,33 @@ public class CallgraphIngestionServiceTests
             return Task.FromResult(document);
         }
     }
+
+    private sealed class StubCallGraphSyncService : ICallGraphSyncService
+    {
+        public CallGraphSyncResult? LastSyncResult { get; private set; }
+        public CallgraphDocument? LastSyncedDocument { get; private set; }
+
+        public Task<CallGraphSyncResult> SyncAsync(
+            Guid scanId,
+            string artifactDigest,
+            CallgraphDocument document,
+            CancellationToken cancellationToken = default)
+        {
+            LastSyncedDocument = document;
+            var result = new CallGraphSyncResult(
+                ScanId: scanId,
+                NodesProjected: document.Nodes.Count,
+                EdgesProjected: document.Edges.Count,
+                EntrypointsProjected: document.Entrypoints.Count,
+                WasUpdated: true,
+                DurationMs: 1);
+            LastSyncResult = result;
+            return Task.FromResult(result);
+        }
+
+        public Task DeleteByScanAsync(Guid scanId, CancellationToken cancellationToken = default)
+        {
+            return Task.CompletedTask;
+        }
+    }
 }