Add valid message schema fixture

Author: Wonderforge-Lab

tests/fixtures/valid/message/record.json

@@ -0,0 +1,20 @@
+{
+  "record_type": "message",
+  "schema_version": 1,
+  "message_id": "20260902-sender-session-fixture",
+  "created_at": "2026-09-02T12:00:00Z",
+  "from_visitor_id": "sender-session",
+  "to_visitor_id": "recipient-session",
+  "to_group": null,
+  "status": "open",
+  "path": "tests/fixtures/valid/message/artifact.md",
+  "tags": [],
+  "reply_to": null,
+  "reply_expected": true,
+  "response_message_id": null,
+  "needs_human_relay": false,
+  "related_packet": null,
+  "related_response": null,
+  "summary": "Valid message fixture.",
+  "notes": ""
+}