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.

samples/provenance/build-statement-sample.json

@@ -0,0 +1,24 @@
+{
+  buildDefinition: {
+    buildType: https://slsa.dev/provenance/v1,
+    externalParameters: {
+      workflow: orchestrator/job,
+      policyHash: sha256:deadbeef
+    },
+    resolvedDependencies: {
+      sbomDigest: sha256:aaaabbbb,
+      vexDigest: sha256:ccccdddd
+    }
+  },
+  buildMetadata: {
+    buildInvocationId: job-12345,
+    buildStartedOn: 2025-11-16T12:00:00Z,
+    buildFinishedOn: 2025-11-16T12:00:10Z,
+    reproducible: true,
+    completeness: {
+      parameters: true,
+      environment: true,
+      materials: true
+    }
+  }
+}