fix(os): strip OpenSearch metadata keys (_id/_score) before write-back

app/stores/opensearch.py

@@ -137,8 +137,11 @@ class OpenSearchStore:
         return _id
 
     async def upsert_doc(self, index: str, doc: dict, doc_id: str) -> str:
+        # Docs read via get_doc carry OpenSearch metadata keys (_id/_score);
+        # writing them back is rejected as metadata fields.
+        clean = {k: v for k, v in doc.items() if not str(k).startswith("_")}
         res = await self.client().put(
-            f"/{self._index(index)}/_doc/{doc_id}", json=doc,
+            f"/{self._index(index)}/_doc/{doc_id}", json=clean,
             params={"refresh": "true"},
         )
         res.raise_for_status()