feat(import): add stable per-resource source keys to the importer contract (#614)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

plugins/importer-postman/tests/fixtures/auth.output.json

@@ -300,5 +300,8 @@
       }
     ],
     "folders": []
+  },
+  "sourceKeys": {
+    "GENERATE_ID::WORKSPACE_0": "collection:9e6dfada-256c-49ea-a38f-7d1b05b7ca2d"
   }
 }

plugins/importer-postman/tests/fixtures/nested.output.json

@@ -88,5 +88,8 @@
         "folderId": "GENERATE_ID::FOLDER_0"
       }
     ]
+  },
+  "sourceKeys": {
+    "GENERATE_ID::WORKSPACE_1": "collection:9e6dfada-256c-49ea-a38f-7d1b05b7ca2d"
   }
 }

plugins/importer-postman/tests/fixtures/params.output.json

@@ -100,5 +100,8 @@
       }
     ],
     "folders": []
+  },
+  "sourceKeys": {
+    "GENERATE_ID::WORKSPACE_2": "collection:9e6dfada-256c-49ea-a38f-7d1b05b7ca2d"
   }
 }