Fix OpenSearch NotFoundError when index doesn't exist (#300)

- Handle opensearchpy.exceptions.NotFoundError in size() method
- Return 0 when index is not found instead of crashing
- Fixes chat functionality when using OpenSearch vector store
- Resolves issue #298

Co-authored-by: hyeongyuy <hyeongyu@hyeongyu-yeo.localdomain>

Author: bigdata-biz

llm-service/app/ai/vector_stores/opensearch.py

@@ -112,7 +112,11 @@ def _find_dim(data_source_id: int) -> int:
 
     def size(self) -> Optional[int]:
         os_client = self._low_level_client
-        return int(os_client.count(index=self.table_name)["count"])
+        try:
+            return int(os_client.count(index=self.table_name)["count"])
+        except opensearchpy.exceptions.NotFoundError:
+            # Return 0 if index doesn't exist yet
+            return 0
 
     def delete(self) -> None:
         os_client = self._low_level_client