fix: datasets documents update-by-file api missing assign field 'indexing_technique' internally (#14243)

api/controllers/service_api/dataset/document.py

 
         if not dataset:
             raise ValueError("Dataset is not exist.")
+
+        # indexing_technique is already set in dataset since this is an update
+        args["indexing_technique"] = dataset.indexing_technique
+
         if "file" in request.files:
             # save file info
             file = request.files["file"]