Complete batch 012 (golden set diff) and 013 (advisory chat), fix build errors

Sprints completed:
- SPRINT_20260110_012_* (golden set diff layer - 10 sprints)
- SPRINT_20260110_013_* (advisory chat - 4 sprints)

Build fixes applied:
- Fix namespace conflicts with Microsoft.Extensions.Options.Options.Create
- Fix VexDecisionReachabilityIntegrationTests API drift (major rewrite)
- Fix VexSchemaValidationTests FluentAssertions method name
- Fix FixChainGateIntegrationTests ambiguous type references
- Fix AdvisoryAI test files required properties and namespace aliases
- Add stub types for CveMappingController (ICveSymbolMappingService)
- Fix VerdictBuilderService static context issue

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Attestor/__Libraries/StellaOps.Attestor.Oci/Services/OrasAttestationAttacher.cs

@@ -203,7 +203,11 @@ public sealed class OrasAttestationAttacher : IOciAttestationAttacher
                 ?? "unknown";
 
             var createdAtStr = referrer.Annotations?.GetValueOrDefault(AnnotationKeys.Created);
-            var createdAt = DateTimeOffset.TryParse(createdAtStr, out var dt)
+            var createdAt = DateTimeOffset.TryParse(
+                createdAtStr,
+                CultureInfo.InvariantCulture,
+                DateTimeStyles.RoundtripKind,
+                out var dt)
                 ? dt
                 : DateTimeOffset.MinValue;