Add invalid response schema fixture

Author: Wonderforge-Lab

tests/fixtures/invalid/response/record.json

@@ -0,0 +1,20 @@
+{
+  "record_type": "response",
+  "schema_version": 1,
+  "response_id": "20260902-review-session-fixture",
+  "created_at": "2026-09-02T12:00:00Z",
+  "created_by": "review-session",
+  "deposited_by": "review-session",
+  "content_origin": "model_generated",
+  "source_refs": [],
+  "derivative_of": [],
+  "provenance_coverage": "record_level",
+  "responding_session": "review-session",
+  "source_packet_id": "20260902-example-session-fixture",
+  "status": "pending_review",
+  "path": "tests/fixtures/invalid/response/artifact.md",
+  "accepted_by": null,
+  "decision_at": null,
+  "tags": [],
+  "notes": "Invalid: model-generated response lacks an upstream record."
+}