fix: skip default image_generation tool injection for gpt-5.3-codex-spark

gpt-5.3-codex-spark is a ChatGPT-account text-only Codex model; the
upstream rejects requests carrying the hosted image_generation tool.
PrepareResponsesBody used to auto-inject the default image tool (and
its bridge instructions) for any text request, so spark requests
failed on the HTTP upstream path where the WS-only stripping from
issue #220 never runs. Guard the auto-injection by model while still
honoring explicitly supplied image tools.

Closes #230

Author: james-6-23

proxy/translator.go

@@ -624,10 +624,23 @@ func hasStructuredResponsesFormat(body map[string]any) bool {
 	return false
 }
 
+// responsesModelRejectsHostedImageTool 判断模型是否不支持 hosted image_generation
+// 工具。gpt-5.3-codex-spark 是 ChatGPT 账号下的纯文本 Codex 模型，上游会直接拒绝
+// 带 hosted 图片工具的请求（issue #230）。
+func responsesModelRejectsHostedImageTool(body map[string]any) bool {
+	model := strings.TrimSpace(firstNonEmptyAnyString(body["model"]))
+	return strings.EqualFold(model, proOnlySparkModel)
+}
+
 func shouldAutoInjectResponsesImageGenerationTool(body map[string]any) bool {
 	if len(body) == 0 || hasResponsesImageGenerationTool(body) {
 		return false
 	}
+	// 不为拒绝 hosted 图片工具的模型自动注入默认图片工具及桥接 instructions；
+	// 用户显式自带的图片工具仍由上面 hasResponsesImageGenerationTool 分支保留。
+	if responsesModelRejectsHostedImageTool(body) {
+		return false
+	}
 	if hasResponsesImageGenerationToolChoice(body) {
 		return true
 	}

proxy/translator_test.go

@@ -944,6 +944,33 @@ func TestPrepareResponsesBody_ToolChoiceImageGenerationAutoInjectsTool(t *testin
 	}
 }
 
+func TestPrepareResponsesBody_SparkModelSkipsDefaultImageGenerationTool(t *testing.T) {
+	raw := []byte(`{"model":"gpt-5.3-codex-spark","input":"hi"}`)
+
+	got, _ := PrepareResponsesBody(raw)
+
+	if gjson.GetBytes(got, "tools").Exists() {
+		t.Fatalf("did not expect default image_generation tool for spark model, got %s", string(got))
+	}
+	if instructions := gjson.GetBytes(got, "instructions").String(); strings.Contains(instructions, codexImageGenerationBridgeMarker) {
+		t.Fatalf("did not expect image generation bridge instructions for spark model, got %q", instructions)
+	}
+}
+
+func TestPrepareResponsesBody_SparkModelKeepsExplicitImageGenerationTool(t *testing.T) {
+	raw := []byte(`{
+		"model":"gpt-5.3-codex-spark",
+		"input":"draw a poster",
+		"tools":[{"type":"image_generation"}]
+	}`)
+
+	got, _ := PrepareResponsesBody(raw)
+
+	if toolType := gjson.GetBytes(got, "tools.0.type").String(); toolType != "image_generation" {
+		t.Fatalf("explicit image_generation tool should be kept for spark model, got %s", string(got))
+	}
+}
+
 func TestPrepareResponsesBody_NormalizesNestedReasoningEffortAliases(t *testing.T) {
 	raw := []byte(`{
 		"model":"gpt-5.4",