fix(grok): 拦截图片模型发往 Responses 端点，返回明确错误

Fixes #4301

backend/internal/service/openai_gateway_grok.go

@@ -49,6 +49,9 @@ func (s *OpenAIGatewayService) forwardGrokResponses(
 	if strings.TrimSpace(upstreamModel) == "" {
 		upstreamModel = grokDefaultResponsesModel
 	}
+	if isGrokImageGenerationModel(upstreamModel) {
+		return nil, fmt.Errorf("model %s is an image model and is not available on the Responses endpoint; use /v1/images/generations instead", upstreamModel)
+	}
 	cacheIdentity := resolveGrokCacheIdentity(c, body, "", upstreamModel)
 	patchedBody, err := patchGrokResponsesBody(body, upstreamModel)
 	if err != nil {

backend/internal/service/openai_gateway_grok_test.go

@@ -2037,3 +2037,25 @@ func TestPatchGrokResponsesBody_MultipleReasoningContentNull(t *testing.T) {
 	require.False(t, items[0].Get("content").Exists())
 	require.False(t, items[2].Get("content").Exists())
 }
+
+func TestIsGrokImageGenerationModel(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		model string
+		want  bool
+	}{
+		{"grok-imagine", true},
+		{"grok-imagine-image-quality", true},
+		{"grok-imagine-edit", true},
+		{"grok-imagine-image-hd", true},
+		{"grok-4.5", false},
+		{"grok-composer", false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.model, func(t *testing.T) {
+			require.Equal(t, tt.want, isGrokImageGenerationModel(tt.model))
+		})
+	}
+}
+