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.Vndr.Chromium/AGENTS.md

@@ -7,7 +7,7 @@ Chromium/Chrome vendor feed connector parsing Stable Channel Update posts; autho
 - Persist raw docs and maintain source_state cursor; idempotent mapping.
 ## Participants
 - Source.Common (HTTP, HTML helpers, validators).
-- Storage.Mongo (document, dto, advisory, alias, affected, reference, psirt_flags, source_state).
+- Storage.Postgres (document, dto, advisory, alias, affected, reference, psirt_flags, source_state).
 - Models (canonical; affected ranges by product/version).
 - Core/WebService (jobs: source:chromium:fetch|parse|map).
 - Merge engine (later) to respect vendor PSIRT precedence for Chrome.
@@ -24,7 +24,7 @@ Out: OS distro packaging semantics; bug bounty details beyond references.
 - Logs: post slugs, version extracted, platform coverage, timing; allowlist blog host.
 ## Tests
 - Author and review coverage in `../StellaOps.Concelier.Connector.Vndr.Chromium.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