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/TaskRunner/StellaOps.TaskRunner/StellaOps.TaskRunner.WebService/OpenApiMetadataFactory.cs

@@ -0,0 +1,38 @@
+using System.Reflection;
+
+namespace StellaOps.TaskRunner.WebService;
+
+internal static class OpenApiMetadataFactory
+{
+    internal static Type ResponseType => typeof(OpenApiMetadata);
+
+    public static OpenApiMetadata Create(string? specUrl = null)
+    {
+        var assembly = Assembly.GetExecutingAssembly().GetName();
+        var version = assembly.Version?.ToString() ?? "0.0.0";
+        var url = string.IsNullOrWhiteSpace(specUrl) ? "/openapi" : specUrl;
+        var etag = CreateWeakEtag(version);
+        var signature = ComputeSignature(url, version);
+
+        return new OpenApiMetadata(url, version, etag, signature);
+    }
+
+    private static string CreateWeakEtag(string input)
+    {
+        if (string.IsNullOrWhiteSpace(input))
+        {
+            input = "0.0.0";
+        }
+
+        return $"W/\"{input}\"";
+    }
+
+    private static string ComputeSignature(string url, string build)
+    {
+        var data = System.Text.Encoding.UTF8.GetBytes(url + build);
+        var hash = System.Security.Cryptography.SHA256.HashData(data);
+        return Convert.ToHexString(hash).ToLowerInvariant();
+    }
+
+    internal sealed record OpenApiMetadata(string Url, string Build, string ETag, string Signature);
+}