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.

offline/notifier/templates/attestation/tmpl-attest-expiry-warning.slack.en-us.template.json

@@ -0,0 +1,16 @@
+{
+  "schemaVersion": "notify.template@1",
+  "templateId": "tmpl-attest-expiry-warning-slack-en-us",
+  "tenantId": "bootstrap",
+  "channelType": "slack",
+  "key": "tmpl-attest-expiry-warning",
+  "locale": "en-us",
+  "renderMode": "markdown",
+  "format": "slack",
+  "description": "Slack reminder for attestations approaching their expiration window.",
+  "body": ":warning: Attestation for `{{payload.subject.digest}}` expires {{expires_in payload.attestation.expiresAt event.ts}}\nRepo: `{{payload.subject.repository}}`{{#if payload.subject.tag}} ({{payload.subject.tag}}){{/if}}\nSigner: `{{fingerprint payload.signer.kid}}` ({{payload.signer.algorithm}})\nIssued: {{payload.attestation.issuedAt}} · Expires: {{payload.attestation.expiresAt}}\nRenewal steps: {{link \"Docs\" payload.links.docs}} · Console: {{link \"Open\" payload.links.console}}\n",
+  "metadata": {
+    "author": "notifications-bootstrap",
+    "version": "2025-11-16"
+  }
+}

offline/notifier/templates/deprecation/tmpl-api-deprecation.email.en-us.template.json

@@ -0,0 +1,16 @@
+{
+  "schemaVersion": "notify.template@1",
+  "templateId": "tmpl-api-deprecation-email-en-us",
+  "tenantId": "bootstrap",
+  "channelType": "email",
+  "key": "tmpl-api-deprecation",
+  "locale": "en-us",
+  "renderMode": "html",
+  "format": "email",
+  "description": "Email notification for retiring Notifier API versions.",
+  "body": "<h2>Notifier API deprecation notice</h2>\n<p>The Notifier API v1 endpoints are scheduled for sunset on <strong>{{metadata.sunset}}</strong>.</p>\n<ul>\n  <li>Paths affected: {{metadata.paths}}</li>\n  <li>Scope: notify.*</li>\n  <li>Replacement: {{metadata.replacement}}</li>\n</ul>\n<p>Action: {{metadata.action}}</p>\n<p>Details: <a href=\"{{metadata.docs}}\">Deprecation bulletin</a></p>\n",
+  "metadata": {
+    "author": "notifications-bootstrap",
+    "version": "2025-11-17"
+  }
+}

offline/notifier/templates/deprecation/tmpl-api-deprecation.slack.en-us.template.json

@@ -0,0 +1,16 @@
+{
+  "schemaVersion": "notify.template@1",
+  "templateId": "tmpl-api-deprecation-slack-en-us",
+  "tenantId": "bootstrap",
+  "channelType": "slack",
+  "key": "tmpl-api-deprecation",
+  "locale": "en-us",
+  "renderMode": "markdown",
+  "format": "slack",
+  "description": "Slack notice for retiring Notifier API versions.",
+  "body": ":warning: Notifier API v1 is being deprecated.\nSunset: {{metadata.sunset}}\nPaths affected: {{metadata.paths}}\nDocs: {{link \"Deprecation details\" metadata.docs}}\nAction: {{metadata.action}}\n",
+  "metadata": {
+    "author": "notifications-bootstrap",
+    "version": "2025-11-17"
+  }
+}