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/ReachGraph/StellaOps.ReachGraph.WebService/Controllers/CveMappingController.cs

@@ -2,6 +2,7 @@
 // Sprint: SPRINT_20260109_009_003_BE_cve_symbol_mapping
 // Task: Implement API endpoints
 
+using System.Collections.Immutable;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.RateLimiting;
 using StellaOps.Reachability.Core.CveMapping;