fix: add missing env var parameter pass through for enable async embedding (#19748)

* Add enable_async parameter to embedding function

* Add enable_async parameter to RAG configuration

backend/open_webui/routers/retrieval.py

@@ -1401,6 +1401,7 @@ def save_docs_to_vector_db(
                 if request.app.state.config.RAG_EMBEDDING_ENGINE == "azure_openai"
                 else None
             ),
+            enable_async=request.app.state.config.ENABLE_ASYNC_EMBEDDING,
         )
 
         # Run async embedding in sync context