Add unit and integration tests for VexCandidateEmitter and SmartDiff repositories

- Implemented comprehensive unit tests for VexCandidateEmitter to validate candidate emission logic based on various scenarios including absent and present APIs, confidence thresholds, and rate limiting.
- Added integration tests for SmartDiff PostgreSQL repositories, covering snapshot storage and retrieval, candidate storage, and material risk change handling.
- Ensured tests validate correct behavior for storing, retrieving, and querying snapshots and candidates, including edge cases and expected outcomes.

src/Concelier/__Libraries/StellaOps.Concelier.Exporter.TrivyDb/AGENTS.md

@@ -9,7 +9,7 @@ Exporter producing a Trivy-compatible database artifact for self-hosting or offl
 - DI: TrivyExporter + Jobs.TrivyExportJob registered by TrivyExporterDependencyInjectionRoutine.
 - Export_state recording: capture digests, counts, start/end timestamps for idempotent reruns and incremental packaging.
 ## Participants
-- Storage.Mongo.AdvisoryStore as input.
+- Storage.Postgres.AdvisoryStore as input.
 - Core scheduler runs export job; WebService/Plugins trigger it.
 - JSON exporter (optional precursor) if choosing the builder path.
 ## Interfaces & contracts
@@ -24,7 +24,7 @@ Out: signing (external pipeline), scanner behavior.
 - Logs: export path, repo/tag, digest; redact credentials; backoff on push errors.
 ## Tests
 - Author and review coverage in `../StellaOps.Concelier.Exporter.TrivyDb.Tests`.
-- Shared fixtures (e.g., `MongoIntegrationFixture`, `ConnectorTestHarness`) live in `../StellaOps.Concelier.Testing`.
+- Shared fixtures (e.g., `PostgresIntegrationFixture`, `ConnectorTestHarness`) live in `../StellaOps.Concelier.Testing`.
 - Keep fixtures deterministic; match new cases to real-world advisories or regression scenarios.