Add unit tests and implementations for MongoDB index models and OpenAPI metadata

- Implemented `MongoIndexModelTests` to verify index models for various stores.
- Created `OpenApiMetadataFactory` with methods to generate OpenAPI metadata.
- Added tests for `OpenApiMetadataFactory` to ensure expected defaults and URL overrides.
- Introduced `ObserverSurfaceSecrets` and `WebhookSurfaceSecrets` for managing secrets.
- Developed `RuntimeSurfaceFsClient` and `WebhookSurfaceFsClient` for manifest retrieval.
- Added dependency injection tests for `SurfaceEnvironmentRegistration` in both Observer and Webhook contexts.
- Implemented tests for secret resolution in `ObserverSurfaceSecretsTests` and `WebhookSurfaceSecretsTests`.
- Created `EnsureLinkNotMergeCollectionsMigrationTests` to validate MongoDB migration logic.
- Added project files for MongoDB tests and NuGet package mirroring.

src/Notifier/StellaOps.Notifier/docs/NOTIFY-OAS-61-ETAG.md

@@ -0,0 +1,15 @@
+# Notifier OAS Discovery — ETag Guidance
+
+The Notifier WebService exposes its OpenAPI document at `/.well-known/openapi` with headers:
+
+- `X-OpenAPI-Scope: notify`
+- `ETag: "<sha256>"` (stable per spec bytes)
+- `Cache-Control: public, max-age=300`
+
+Usage notes:
+
+- SDK generators and CI smoke tests should re-use the `ETag` for conditional GETs (`If-None-Match`) to avoid redundant downloads.
+- Mirror/Offline bundles should copy `openapi/notify-openapi.yaml` and retain the `ETag` alongside the file hash used in air-gap validation.
+- When the spec changes, the SHA-256 and `ETag` change together; callers can detect breaking/non-breaking updates via the published changelog (source of truth in `docs/api/notify-openapi.yaml`).
+
+Applies to tasks: NOTIFY-OAS-61-001/61-002/63-001.