chat : fix gemma4 required tool grammar (#29115)

common/parsers/gemma4.cpp

@@ -272,6 +272,10 @@ common_chat_params common_chat_params_init_gemma4(const common_chat_template &
                 /* max = */ inputs.parallel_tool_calls ? -1 : 1
             ));
 
+            if (inputs.tool_choice == COMMON_CHAT_TOOL_CHOICE_REQUIRED) {
+                return start + thought + tool_call;
+            }
+
             auto scan_to_toolcall = p.rule("scan-to-toolcall", p.until("<|tool_call>"));
             auto content = p.rule("content", p.content(p.until_one_of({"<|channel>", "<channel|>", "<|tool_call>"})));
             auto message = p.rule("message", thought + content);

tests/test-chat.cpp

@@ -3032,6 +3032,26 @@ static void test_template_output_peg_parsers(bool detailed_debug) {
             .expect(message_with_content_and_tool_call("Hello, world!\nWhat's up?", "get_time", R"({"city": "Paris"})"))
             .run();
 
+        // Required tool call
+        tst.test(
+                "<|tool_call>call:get_time{city:<|\"|>Paris<|\"|>}<tool_call|>")
+            .tools({ get_time_tool })
+            .tool_choice(COMMON_CHAT_TOOL_CHOICE_REQUIRED)
+            .expect(message_with_tool_calls("get_time", R"({"city": "Paris"})"))
+            .run();
+
+        // Required tool call after reasoning
+        tst.test(
+                "<|channel>thought\nI'm\nthinking<channel|><|tool_call>call:get_time{city:<|\"|>Paris<|\"|>}<tool_call|>")
+            .reasoning_format(COMMON_REASONING_FORMAT_AUTO)
+            .tools({ get_time_tool })
+            .tool_choice(COMMON_CHAT_TOOL_CHOICE_REQUIRED)
+            .expect_reasoning("I'm\nthinking")
+            .expect_tool_calls({
+                { "get_time", R"({"city": "Paris"})", {} },
+            })
+            .run();
+
         // Parallel tool calls
         tst.test(
                 "<|tool_call>call:get_time{city:<|\"|>London<|\"|>}<tool_call|>"