test: update example messages to match new schema

test/mock/messages/Message_VehicleMission.json

 {
-  "message_type": "VehicleMission",
   "metadata": {
     "source": "ae",
     "destination": "soar.po.ecosub.eco1",
     "delivery_type": "publish",
     "message_id": "test"
   },
+  "payload": {
+    "message_type": "VehicleMission",
     "operator_id": "po",
     "vehicle_id": "eco1",
     "coordinates": {
@@ -14,4 +15,5 @@
       "depth": 50,
       "projection": 4326
     }
+  }
 }

test/mock/messages/Message_VehicleMissionActions.json

 {
-  "message_type": "VehicleMission",
   "metadata": {
     "source": "ae",
     "destination": "soar.po.ecosub.eco1",
     "delivery_type": "publish",
     "message_id": "test"
   },
+  "payload": {
+    "message_type": "VehicleMission",
     "operator_id": "po",
     "vehicle_id": "eco1",
     "coordinates": {
@@ -34,4 +35,5 @@
         }
       }
     ]
+  }
 }

test/mock/messages/Message_VehicleStatus.json

 {
-  "message_type": "VehicleStatus",
   "metadata": {
     "source": "ae",
     "destination": "soar.po.ecosub.eco1",
     "delivery_type": "publish",
     "message_id": "test"
   },
+  "payload": {
+    "message_type": "VehicleStatus",
     "operator_id": "po",
     "vehicle_id": "eco1",
     "coordinates": {
@@ -15,4 +16,5 @@
       "projection": 4326
     },
     "battery_percentage": 64
+  }
 }
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