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/10_PLUGIN_SDK_GUIDE.md

@@ -101,7 +101,7 @@ using StellaOps.DependencyInjection;
 [ServiceBinding(typeof(IJob), ServiceLifetime.Scoped, RegisterAsSelf = true)]
 public sealed class MyJob : IJob
 {
-    // IJob dependencies can now use scoped services (Mongo sessions, etc.)
+    // IJob dependencies can now use scoped services (PostgreSQL connections, etc.)
 }
 ~~~
 
@@ -216,7 +216,7 @@ On merge, the plug‑in shows up in the UI Marketplace.
 | NotDetected         | .sig missing               | cosign sign …                               |
 | VersionGateMismatch | Backend 2.1 vs plug‑in 2.0 | Re‑compile / bump attribute                 |
 | FileLoadException   | Duplicate                  | StellaOps.Common	Ensure PrivateAssets="all" |
-| Redis               | timeouts	Large writes      | Batch or use Mongo                          |
+| Redis               | timeouts	Large writes      | Batch or use PostgreSQL                     |
 
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