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.Tests/AttestationTemplateCoverageTests.cs

@@ -0,0 +1,77 @@
+using System.Text.Json;
+using Xunit;
+
+namespace StellaOps.Notifier.Tests;
+
+public sealed class AttestationTemplateCoverageTests
+{
+    private static readonly string RepoRoot = LocateRepoRoot();
+
+    [Fact]
+    public void Attestation_templates_cover_required_channels()
+    {
+        var directory = Path.Combine(RepoRoot, "offline", "notifier", "templates", "attestation");
+        Assert.True(Directory.Exists(directory), $"Expected template directory at {directory}");
+
+        var templates = Directory
+            .GetFiles(directory, "*.template.json")
+            .Select(path => new
+            {
+                Path = path,
+                Document = JsonDocument.Parse(File.ReadAllText(path)).RootElement
+            })
+            .ToList();
+
+        var required = new Dictionary<string, string[]>
+        {
+            ["tmpl-attest-verify-fail"] = new[] { "slack", "email", "webhook" },
+            ["tmpl-attest-expiry-warning"] = new[] { "email", "slack" },
+            ["tmpl-attest-key-rotation"] = new[] { "email", "webhook" },
+            ["tmpl-attest-transparency-anomaly"] = new[] { "slack", "webhook" }
+        };
+
+        foreach (var pair in required)
+        {
+            var matches = templates.Where(t => t.Document.GetProperty("key").GetString() == pair.Key);
+            var channels = matches
+                .Select(t => t.Document.GetProperty("channelType").GetString() ?? string.Empty)
+                .ToHashSet(StringComparer.OrdinalIgnoreCase);
+
+            var missing = pair.Value.Where(requiredChannel => !channels.Contains(requiredChannel)).ToArray();
+            Assert.True(missing.Length == 0, $"{pair.Key} missing channels: {string.Join(", ", missing)}");
+        }
+    }
+
+    [Fact]
+    public void Attestation_templates_include_schema_and_locale_metadata()
+    {
+        var directory = Path.Combine(RepoRoot, "offline", "notifier", "templates", "attestation");
+        Assert.True(Directory.Exists(directory), $"Expected template directory at {directory}");
+
+        foreach (var path in Directory.GetFiles(directory, "*.template.json"))
+        {
+            var document = JsonDocument.Parse(File.ReadAllText(path)).RootElement;
+
+            Assert.True(document.TryGetProperty("schemaVersion", out var schemaVersion) && !string.IsNullOrWhiteSpace(schemaVersion.GetString()), $"schemaVersion missing for {Path.GetFileName(path)}");
+            Assert.True(document.TryGetProperty("locale", out var locale) && !string.IsNullOrWhiteSpace(locale.GetString()), $"locale missing for {Path.GetFileName(path)}");
+            Assert.True(document.TryGetProperty("key", out var key) && !string.IsNullOrWhiteSpace(key.GetString()), $"key missing for {Path.GetFileName(path)}");
+        }
+    }
+
+    private static string LocateRepoRoot()
+    {
+        var directory = AppContext.BaseDirectory;
+        while (directory != null)
+        {
+            var candidate = Path.Combine(directory, "offline", "notifier", "templates", "attestation");
+            if (Directory.Exists(candidate))
+            {
+                return directory;
+            }
+
+            directory = Directory.GetParent(directory)?.FullName;
+        }
+
+        throw new InvalidOperationException("Unable to locate repository root containing offline/notifier/templates/attestation.");
+    }
+}