set thinking field for tool use

Author: aldehir

common/chat.cpp

@@ -297,6 +297,9 @@ json common_chat_msgs_to_json_oaicompat(const std::vector<common_chat_msg> & msg
         }
         if (!msg.reasoning_content.empty()) {
             jmsg["reasoning_content"] = msg.reasoning_content;
+
+            // gpt-oss requires the thinking field for tool calls
+            jmsg["thinking"] = msg.reasoning_content;
         }
         if (!msg.tool_name.empty()) {
             jmsg["name"] = msg.tool_name;