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.Distro.RedHat/AGENTS.md

@@ -7,7 +7,7 @@ Red Hat distro connector (Security Data API and OVAL) providing authoritative OS
 - Map to canonical advisories with affected Type=rpm/cpe, fixedBy NEVRA, RHSA aliasing; persist provenance indicating oval/package.nevra.
 ## Participants
 - Source.Common (HTTP, throttling, validators).
-- Storage.Mongo (document, dto, advisory, alias, affected, reference, source_state).
+- Storage.Postgres (document, dto, advisory, alias, affected, reference, source_state).
 - Models (canonical Affected with NEVRA).
 - Core/WebService (jobs: source:redhat:fetch|parse|map) already registered.
 - Merge engine to enforce distro precedence (OVAL or PSIRT greater than NVD).
@@ -23,7 +23,7 @@ Out: building RPM artifacts; cross-distro reconciliation beyond Red Hat.
 - Logs: cursor bounds, advisory ids, NEVRA counts; allowlist Red Hat endpoints.
 ## Tests
 - Author and review coverage in `../StellaOps.Concelier.Connector.Distro.RedHat.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