tool-calls: allow r1 output to miss <think> opening tag (since latest template update adds it)

Author: ochafik

common/chat.cpp

@@ -640,7 +640,7 @@ static common_chat_params common_chat_params_init_deepseek_r1(const common_chat_
 static common_chat_msg common_chat_parse_deepseek_r1(const std::string & input, bool extract_reasoning) {
     static std::regex function_regex("<｜tool▁call▁begin｜>function<｜tool▁sep｜>([^\n]+)\n```json\n");
     static std::regex close_regex("```[\\s\\r\\n]*<｜tool▁call▁end｜>");
-    static std::regex reasoning_content_regex("(<think>([\\s\\S\\r\\n]*?)</think>)?([\\s\\S\\r\\n]*)");
+    static std::regex reasoning_content_regex("((?:<think>)?([\\s\\S\\r\\n]*?)</think>)?([\\s\\S\\r\\n]*)");
     static std::regex tool_calls_regex("[\\s\\r\\n]*(?:<｜tool▁calls▁begin｜>|<｜tool_calls_begin｜>|<｜tool calls begin｜>|<｜tool\\\\_calls\\\\_begin｜>)([\\s\\S\\r\\n]*?)<｜tool▁calls▁end｜>");
     common_chat_msg msg;
     msg.role = "assistant";

tests/test-chat.cpp

@@ -675,6 +675,10 @@ static void test_template_output_parsers() {
         assert_msg_equals(msg_from_json(message_assist_thoughts),
             common_chat_parse("<think>I'm thinking</think>Hello, world!\nWhat's up?",
             COMMON_CHAT_FORMAT_DEEPSEEK_R1_EXTRACT_REASONING));
+        assert_msg_equals(msg_from_json(message_assist_thoughts),
+            // Latest template update (ast of 20250209) adds a trailing <think>\n if add_generation_prompt is true.
+            common_chat_parse("I'm thinking</think>Hello, world!\nWhat's up?",
+            COMMON_CHAT_FORMAT_DEEPSEEK_R1_EXTRACT_REASONING));
         // test_template(tmpl, end_tokens, message_assist_call, tools,
         //               "<｜tool▁calls▁begin｜><｜tool▁call▁begin｜>function<｜tool▁sep｜>special_function\n"
         //               "```json\n"