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/Provenance/__Tests/StellaOps.Provenance.Attestation.Tests/PromotionAttestationBuilderTests.cs

@@ -0,0 +1,26 @@
+using System.Text;
+using FluentAssertions;
+using StellaOps.Provenance.Attestation;
+using Xunit;
+
+namespace StellaOps.Provenance.Attestation.Tests;
+
+public class PromotionAttestationBuilderTests
+{
+    [Fact]
+    public void Produces_canonical_json_for_predicate()
+    {
+        var predicate = new PromotionPredicate(
+            ImageDigest: sha256:img,
+            SbomDigest: sha256:sbom,
+            VexDigest: sha256:vex,
+            PromotionId: prom-1,
+            RekorEntry: uuid,
+            Metadata: new Dictionary<string, string>{{env,prod}});
+
+        var bytes = PromotionAttestationBuilder.CreateCanonicalJson(predicate);
+        var json = Encoding.UTF8.GetString(bytes);
+
+        json.Should().Be("ImageDigest":"sha256:img");
+    }
+}