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.Connector.Common/AGENTS.md

@@ -10,7 +10,7 @@ Shared connector toolkit. Provides HTTP clients, retry/backoff, conditional GET
 - HTML sanitization, URL normalization, and PDF-to-text extraction utilities for feeds that require cleanup before validation.
 ## Participants
 - Source.* connectors (NVD, Red Hat, JVN, PSIRTs, CERTs, ICS).
-- Storage.Mongo (document/dto repositories using shared shapes).
+- Storage.Postgres (document/dto repositories using shared shapes).
 - Core (jobs schedule/trigger for connectors).
 - QA (canned HTTP server harness, schema fixtures).
 ## Interfaces & contracts
@@ -27,7 +27,7 @@ Out: connector-specific schemas/mapping rules, merge precedence.
 - Distributed tracing hooks and per-connector counters should be wired centrally for consistent observability.
 ## Tests
 - Author and review coverage in `../StellaOps.Concelier.Connector.Common.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.
 
 ## Required Reading