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/StellaOps.Notifier.WebService/Setup/OpenApiDocumentCache.cs

@@ -0,0 +1,28 @@
+using System.Text;
+
+namespace StellaOps.Notifier.WebService.Setup;
+
+public sealed class OpenApiDocumentCache
+{
+    private readonly string _document;
+    private readonly string _hash;
+
+    public OpenApiDocumentCache(IHostEnvironment environment)
+    {
+        var path = Path.Combine(environment.ContentRootPath, "openapi", "notify-openapi.yaml");
+        if (!File.Exists(path))
+        {
+            throw new FileNotFoundException("OpenAPI document not found.", path);
+        }
+
+        _document = File.ReadAllText(path, Encoding.UTF8);
+
+        using var sha = System.Security.Cryptography.SHA256.Create();
+        var bytes = Encoding.UTF8.GetBytes(_document);
+        _hash = Convert.ToHexString(sha.ComputeHash(bytes)).ToLowerInvariant();
+    }
+
+    public string Document => _document;
+
+    public string Sha256 => _hash;
+}

src/Notifier/StellaOps.Notifier/StellaOps.Notifier.WebService/Setup/WebServiceAssemblyMarker.cs

@@ -0,0 +1,6 @@
+namespace StellaOps.Notifier.WebService;
+
+/// <summary>
+/// Marker type used for testing/hosting the web application.
+/// </summary>
+public sealed class WebServiceAssemblyMarker;