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.

docs/advisory-ai/architecture.md

@@ -27,7 +27,7 @@ Conseiller / Excititor / SBOM / Policy
             v
    +----------------------------+
    | Cache & Provenance          |
-   |   (Mongo + DSSE optional)   |
+   |   (PostgreSQL + DSSE opt.)  |
    +----------------------------+
             |                 \
             v                  v
@@ -48,7 +48,7 @@ Key stages:
 | `AdvisoryPipelineOrchestrator` | Builds task plans, selects prompt templates, allocates token budgets. | Tenant-scoped; memoises by cache key. |
 | `GuardrailService` | Applies redaction filters, prompt allowlists, validation schemas, and DSSE sealing. | Shares configuration with Security Guild. |
 | `ProfileRegistry` | Maps profile IDs to runtime implementations (local model, remote connector). | Enforces tenant consent and allowlists. |
-| `AdvisoryOutputStore` | Mongo collection storing cached artefacts plus provenance manifest. | TTL defaults 24h; DSSE metadata optional. |
+| `AdvisoryOutputStore` | PostgreSQL table storing cached artefacts plus provenance manifest. | TTL defaults 24h; DSSE metadata optional. |
 | `AdvisoryPipelineWorker` | Background executor for queued jobs (future sprint once 004A wires queue). | Consumes `advisory.pipeline.execute` messages. |
 
 ## 3. Data contracts