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/Zastava/__Tests/StellaOps.Zastava.Observer.Tests/Surface/RuntimeSurfaceFsClientTests.cs

@@ -0,0 +1,58 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using StellaOps.Scanner.Surface.FS;
+using StellaOps.Zastava.Observer.Configuration;
+using StellaOps.Zastava.Observer.Surface;
+using Xunit;
+
+namespace StellaOps.Zastava.Observer.Tests.Surface;
+
+public sealed class RuntimeSurfaceFsClientTests
+{
+    [Fact]
+    public async Task TryGetManifestAsync_ReturnsPublishedManifest()
+    {
+        var configuration = new ConfigurationBuilder()
+            .AddInMemoryCollection(new Dictionary<string, string?>
+            {
+                [$"{ZastavaObserverOptions.SectionName}:runtimes:0:engine"] = "Containerd",
+                [$"{ZastavaObserverOptions.SectionName}:backend:baseAddress"] = "https://scanner.internal"
+            })
+            .Build();
+
+        var services = new ServiceCollection();
+        services.AddLogging();
+        services.AddZastavaObserver(configuration);
+
+        using var provider = services.BuildServiceProvider();
+        var manifestWriter = provider.GetRequiredService<ISurfaceManifestWriter>();
+        var client = provider.GetRequiredService<IRuntimeSurfaceFsClient>();
+
+        var document = new SurfaceManifestDocument
+        {
+            Tenant = "default",
+            ImageDigest = "sha256:deadbeef",
+            GeneratedAt = DateTimeOffset.UtcNow,
+            Artifacts = new[]
+            {
+                new SurfaceManifestArtifact
+                {
+                    Kind = "entry-trace",
+                    Uri = "cas://surface-cache/manifest/entry-trace",
+                    Digest = "sha256:abc123",
+                    MediaType = "application/json",
+                    Format = "ndsjon"
+                }
+            }
+        };
+
+        var published = await manifestWriter.PublishAsync(document, default);
+        var fetched = await client.TryGetManifestAsync(published.ManifestDigest, default);
+
+        Assert.NotNull(fetched);
+        Assert.Equal(document.Tenant, fetched!.Tenant);
+        Assert.Equal(document.ImageDigest, fetched.ImageDigest);
+    }
+}