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.Tests/MongoIndexModelTests.cs

@@ -0,0 +1,62 @@
+using MongoDB.Driver;
+using StellaOps.TaskRunner.Infrastructure.Execution;
+using Xunit;
+
+namespace StellaOps.TaskRunner.Tests;
+
+public sealed class MongoIndexModelTests
+{
+    [Fact]
+    public void StateStore_indexes_match_contract()
+    {
+        var models = MongoPackRunStateStore.GetIndexModels().ToArray();
+
+        Assert.Collection(models,
+            model => Assert.Equal("pack_runs_updatedAt_desc", model.Options.Name),
+            model => Assert.Equal("pack_runs_tenant_updatedAt_desc", model.Options.Name));
+
+        Assert.True(models[1].Options.Sparse ?? false);
+    }
+
+    [Fact]
+    public void LogStore_indexes_match_contract()
+    {
+        var models = MongoPackRunLogStore.GetIndexModels().ToArray();
+
+        Assert.Collection(models,
+            model =>
+            {
+                Assert.Equal("pack_run_logs_run_sequence", model.Options.Name);
+                Assert.True(model.Options.Unique ?? false);
+            },
+            model => Assert.Equal("pack_run_logs_run_timestamp", model.Options.Name));
+    }
+
+    [Fact]
+    public void ArtifactStore_indexes_match_contract()
+    {
+        var models = MongoPackRunArtifactUploader.GetIndexModels().ToArray();
+
+        Assert.Collection(models,
+            model =>
+            {
+                Assert.Equal("pack_artifacts_run_name", model.Options.Name);
+                Assert.True(model.Options.Unique ?? false);
+            },
+            model => Assert.Equal("pack_artifacts_run", model.Options.Name));
+    }
+
+    [Fact]
+    public void ApprovalStore_indexes_match_contract()
+    {
+        var models = MongoPackRunApprovalStore.GetIndexModels().ToArray();
+
+        Assert.Collection(models,
+            model =>
+            {
+                Assert.Equal("pack_run_approvals_run_approval", model.Options.Name);
+                Assert.True(model.Options.Unique ?? false);
+            },
+            model => Assert.Equal("pack_run_approvals_run_status", model.Options.Name));
+    }
+}