gpt-oss : fix ungrouped tool calls in grammar

Author: aldehir

common/chat.cpp

@@ -1369,13 +1369,13 @@ static common_chat_params common_chat_params_init_gpt_oss(const common_chat_temp
             });
 
             auto recipient_in_role = builder.add_rule("recipient_in_role",
-                "\"<|start|>assistant\"? \" to=functions.\" " +
-                string_join(tool_rules_recipient_in_role, " | ")
+                "\"<|start|>assistant\"? \" to=functions.\" ( " +
+                string_join(tool_rules_recipient_in_role, " | ") + " )"
             );
 
             auto recipient_in_channel = builder.add_rule("recipient_in_channel",
-                channel + " \" to=functions.\" " +
-                string_join(tool_rules_recipient_in_channel, " | ")
+                channel + " \" to=functions.\" ( " +
+                string_join(tool_rules_recipient_in_channel, " | ") + " )"
             );
 
             builder.add_rule("root", recipient_in_role + " | " + recipient_in_channel);