test(translator): add tests to validate omission of top-level output_text in OpenAI responses

- Added `TestConvertOpenAIChatCompletionsResponseToOpenAIResponses_CompletedOmitsTopLevelOutputText` to ensure `output_text` is excluded in streamed responses.
- Added `TestConvertOpenAIChatCompletionsResponseToOpenAIResponses_ToolCallCompletedOmitsTopLevelOutputText` to validate behavior during tool call completions.
- Introduced `TestConvertOpenAIChatCompletionsResponseToOpenAIResponsesNonStream_OmitsTopLevelOutputText` to confirm the omission of `output_text` in non-streamed responses.
- Expanded test coverage to ensure consistency with native OpenAI responses.

internal/translator/openai/openai/responses/openai_openai-responses_response_test.go

@@ -371,6 +371,72 @@ func TestConvertOpenAIChatCompletionsResponseToOpenAIResponses_MixedMessageAndTo
 	}
 }
 
+func TestConvertOpenAIChatCompletionsResponseToOpenAIResponses_CompletedOmitsTopLevelOutputText(t *testing.T) {
+	in := []string{
+		`data: {"id":"resp_output_text","object":"chat.completion.chunk","created":1773896263,"model":"model","choices":[{"index":0,"delta":{"role":"assistant","content":"hello ","reasoning_content":null,"tool_calls":null},"finish_reason":null}]}`,
+		`data: {"id":"resp_output_text","object":"chat.completion.chunk","created":1773896263,"model":"model","choices":[{"index":0,"delta":{"role":null,"content":"world","reasoning_content":null,"tool_calls":null},"finish_reason":"stop"}],"usage":{"completion_tokens":2,"total_tokens":4,"prompt_tokens":2}}`,
+		`data: [DONE]`,
+	}
+
+	request := []byte(`{"model":"gpt-5.4"}`)
+
+	var param any
+	var completed gjson.Result
+	for _, line := range in {
+		for _, chunk := range ConvertOpenAIChatCompletionsResponseToOpenAIResponses(context.Background(), "model", request, request, []byte(line), &param) {
+			ev, data := parseOpenAIResponsesSSEEvent(t, chunk)
+			if ev == "response.completed" {
+				completed = data
+			}
+		}
+	}
+
+	if !completed.Exists() {
+		t.Fatal("expected response.completed event")
+	}
+	if completed.Get("response.output_text").Exists() {
+		t.Fatalf("response.output_text should be omitted to match native Responses output: %s", completed.Get("response.output_text").Raw)
+	}
+	if got := completed.Get("response.output.0.content.0.text").String(); got != "hello world" {
+		t.Fatalf("response.output text = %q, want %q", got, "hello world")
+	}
+}
+
+func TestConvertOpenAIChatCompletionsResponseToOpenAIResponses_ToolCallCompletedOmitsTopLevelOutputText(t *testing.T) {
+	in := []string{
+		`data: {"id":"resp_tool_output_text","object":"chat.completion.chunk","created":1773896263,"model":"model","choices":[{"index":0,"delta":{"role":"assistant","content":"I will call the weather tool.","reasoning_content":null,"tool_calls":null},"finish_reason":null}]}`,
+		`data: {"id":"resp_tool_output_text","object":"chat.completion.chunk","created":1773896263,"model":"model","choices":[{"index":0,"delta":{"role":"assistant","content":null,"reasoning_content":null,"tool_calls":[{"index":0,"id":"call_weather","type":"function","function":{"name":"get_weather","arguments":""}}]},"finish_reason":null}]}`,
+		`data: {"id":"resp_tool_output_text","object":"chat.completion.chunk","created":1773896263,"model":"model","choices":[{"index":0,"delta":{"role":null,"content":null,"reasoning_content":null,"tool_calls":[{"index":0,"function":{"arguments":"{\"location\":\"北京\",\"unit\":\"celsius\"}"}}]},"finish_reason":"tool_calls"}],"usage":{"completion_tokens":10,"total_tokens":20,"prompt_tokens":10}}`,
+		`data: [DONE]`,
+	}
+
+	request := []byte(`{"model":"gpt-5.4","tool_choice":"auto","parallel_tool_calls":true}`)
+
+	var param any
+	var completed gjson.Result
+	for _, line := range in {
+		for _, chunk := range ConvertOpenAIChatCompletionsResponseToOpenAIResponses(context.Background(), "model", request, request, []byte(line), &param) {
+			ev, data := parseOpenAIResponsesSSEEvent(t, chunk)
+			if ev == "response.completed" {
+				completed = data
+			}
+		}
+	}
+
+	if !completed.Exists() {
+		t.Fatal("expected response.completed event")
+	}
+	if completed.Get("response.output_text").Exists() {
+		t.Fatalf("response.output_text should be omitted to match native Responses output: %s", completed.Get("response.output_text").Raw)
+	}
+	if got := completed.Get("response.output.0.content.0.text").String(); got != "I will call the weather tool." {
+		t.Fatalf("response output text = %q, want %q", got, "I will call the weather tool.")
+	}
+	if got := completed.Get("response.output.1.arguments").String(); !strings.Contains(got, "北京") {
+		t.Fatalf("response function call arguments = %q, want Beijing argument", got)
+	}
+}
+
 func TestConvertOpenAIChatCompletionsResponseToOpenAIResponses_FunctionCallDoneAndCompletedOutputStayAscending(t *testing.T) {
 	in := []string{
 		`data: {"id":"resp_order","object":"chat.completion.chunk","created":1773896263,"model":"model","choices":[{"index":0,"delta":{"role":"assistant","content":null,"reasoning_content":null,"tool_calls":[{"index":0,"id":"call_glob","type":"function","function":{"name":"glob","arguments":""}}]},"finish_reason":null}]}`,
@@ -421,3 +487,18 @@ func TestConvertOpenAIChatCompletionsResponseToOpenAIResponses_FunctionCallDoneA
 		t.Fatalf("unexpected completed function_call order: %v", completedOrder)
 	}
 }
+
+func TestConvertOpenAIChatCompletionsResponseToOpenAIResponsesNonStream_OmitsTopLevelOutputText(t *testing.T) {
+	request := []byte(`{"model":"gpt-5.4"}`)
+	raw := []byte(`{"id":"chatcmpl_output_text","object":"chat.completion","created":1773896263,"model":"model","choices":[{"index":0,"message":{"role":"assistant","content":"ping"},"finish_reason":"stop"}],"usage":{"prompt_tokens":2,"completion_tokens":1,"total_tokens":3}}`)
+
+	resp := ConvertOpenAIChatCompletionsResponseToOpenAIResponsesNonStream(context.Background(), "model", request, request, raw, nil)
+	data := gjson.ParseBytes(resp)
+
+	if data.Get("output_text").Exists() {
+		t.Fatalf("output_text should be omitted to match native Responses output: %s", resp)
+	}
+	if got := data.Get("output.0.content.0.text").String(); got != "ping" {
+		t.Fatalf("output text = %q, want %q; response=%s", got, "ping", resp)
+	}
+}