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/Notifier/StellaOps.Notifier/StellaOps.Notifier.WebService/Contracts/PackApprovalRequest.cs

@@ -0,0 +1,45 @@
+using System.Text.Json.Serialization;
+
+namespace StellaOps.Notifier.WebService.Contracts;
+
+public sealed class PackApprovalRequest
+{
+    [JsonPropertyName("eventId")]
+    public Guid EventId { get; init; }
+
+    [JsonPropertyName("issuedAt")]
+    public DateTimeOffset IssuedAt { get; init; }
+
+    [JsonPropertyName("kind")]
+    public string Kind { get; init; } = string.Empty;
+
+    [JsonPropertyName("packId")]
+    public string PackId { get; init; } = string.Empty;
+
+    [JsonPropertyName("policy")]
+    public PackApprovalPolicy? Policy { get; init; }
+
+    [JsonPropertyName("decision")]
+    public string Decision { get; init; } = string.Empty;
+
+    [JsonPropertyName("actor")]
+    public string Actor { get; init; } = string.Empty;
+
+    [JsonPropertyName("resumeToken")]
+    public string? ResumeToken { get; init; }
+
+    [JsonPropertyName("summary")]
+    public string? Summary { get; init; }
+
+    [JsonPropertyName("labels")]
+    public Dictionary<string, string>? Labels { get; init; }
+}
+
+public sealed class PackApprovalPolicy
+{
+    [JsonPropertyName("id")]
+    public string? Id { get; init; }
+
+    [JsonPropertyName("version")]
+    public string? Version { get; init; }
+}