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.

docs/samples/lnm/linkset-ghsa.json

@@ -0,0 +1,20 @@
+{
+  "_id": "0000000000000000000000aa",
+  "tenantId": "demo-tenant",
+  "source": "ghsa",
+  "advisoryId": "GHSA-xxxx-yyyy",
+  "observations": [ "000000000000000000000001" ],
+  "normalized": {
+    "purls": [ "pkg:npm/example" ],
+    "versions": [ "1.2.3" ],
+    "ranges": [ { "type": "semver", "events": [ { "introduced": "0" }, { "fixed": "1.2.4" } ] } ],
+    "severities": [ { "system": "cvssv3.1", "score": 7.5, "vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N" } ]
+  },
+  "createdAt": "2025-10-06T12:05:00Z",
+  "builtByJobId": "linkset-builder-456",
+  "provenance": {
+    "observationHashes": [ "sha256:abc123" ],
+    "toolVersion": "lnm-1.0.0",
+    "policyHash": "sha256:def456"
+  }
+}

docs/samples/lnm/observation-ghsa.json

@@ -0,0 +1,24 @@
+{
+  "_id": "000000000000000000000001",
+  "tenantId": "demo-tenant",
+  "source": "ghsa",
+  "advisoryId": "GHSA-xxxx-yyyy",
+  "title": "Example GHSA vuln",
+  "summary": "Example summary",
+  "severities": [ { "system": "cvssv3.1", "score": 7.5, "vector": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N" } ],
+  "affected": [ {
+    "purl": "pkg:npm/example@1.2.3",
+    "versions": [ "1.2.3" ],
+    "ranges": [ { "type": "semver", "events": [ { "introduced": "0" }, { "fixed": "1.2.4" } ] } ]
+  } ],
+  "references": [ "https://github.com/example/advisory" ],
+  "weaknesses": [ "CWE-79" ],
+  "published": "2025-10-01T00:00:00Z",
+  "modified": "2025-10-05T00:00:00Z",
+  "provenance": {
+    "sourceArtifactSha": "sha256:abc123",
+    "fetchedAt": "2025-10-06T12:00:00Z",
+    "ingestJobId": "ingest-123"
+  },
+  "ingestedAt": "2025-10-06T12:01:00Z"
+}