fix(acceptance): 校准 Seedance 图片转换验收

按 Volces 官方输入边界补齐 Seedance 2.0 候选能力，将错误的合法 4K 转换样本替换为真实越界图片，并让协议模拟器校验物化后的 Base64 data URL。\n\n验证：Go 全量测试、迁移安全检查、gofmt。

apps/api/internal/acceptanceemulator/server_test.go

@@ -58,14 +58,14 @@ func TestGeminiAndVolcesProtocolEmulation(t *testing.T) {
 	}
 	_ = response.Body.Close()
 
-	fixtureResponse, err := http.Get(httpServer.URL + "/fixtures/image-12-4k.jpg")
+	fixtureResponse, err := http.Get(httpServer.URL + "/fixtures/image-12-oversized.jpg")
 	if err != nil {
-		t.Fatalf("get 4K fixture: %v", err)
+		t.Fatalf("get oversized fixture: %v", err)
 	}
 	config, format, err := image.DecodeConfig(fixtureResponse.Body)
 	_ = fixtureResponse.Body.Close()
-	if err != nil || format != "jpeg" || config.Width != 4096 || config.Height != 2160 {
-		t.Fatalf("4K fixture format=%s config=%+v err=%v", format, config, err)
+	if err != nil || format != "jpeg" || config.Width != 6144 || config.Height != 2160 {
+		t.Fatalf("oversized fixture format=%s config=%+v err=%v", format, config, err)
 	}
 
 	content := []any{map[string]any{"type": "text", "text": "video"}}
@@ -125,6 +125,29 @@ func TestGeminiAndVolcesProtocolEmulation(t *testing.T) {
 	}
 }
 
+func TestForcedConversionRejectsImagesOutsideOfficialSeedanceRange(t *testing.T) {
+	server := New(Config{Wait: func(context.Context, time.Duration) error { return nil }})
+	httpServer := httptest.NewServer(server.Handler())
+	defer httpServer.Close()
+
+	content := []any{map[string]any{"type": "text", "text": "acceptance-force-conversion"}}
+	for _, name := range []string{"image-12-oversized.jpg", "image-00.png", "image-04.jpg"} {
+		content = append(content, map[string]any{
+			"type": "image_url", "role": "reference_image",
+			"image_url": map[string]any{"url": httpServer.URL + "/fixtures/" + name},
+		})
+	}
+	body, _ := json.Marshal(map[string]any{"model": "seedance-test", "content": content})
+	response, err := postIdempotent(httpServer.URL+"/contents/generations/tasks", body, "oversized")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer response.Body.Close()
+	if response.StatusCode != http.StatusBadRequest {
+		t.Fatalf("status=%d, want %d", response.StatusCode, http.StatusBadRequest)
+	}
+}
+
 func TestPaddedPNGIsValidAndExact(t *testing.T) {
 	for _, size := range []int{256 << 10, 4 << 20, 8 << 20} {
 		payload := paddedPNG(size)
@@ -148,6 +171,7 @@ func TestVolcesProtocolAcceptsThreeSixAndNineReferenceImages(t *testing.T) {
 	server := New(Config{Wait: func(context.Context, time.Duration) error { return nil }})
 	httpServer := httptest.NewServer(server.Handler())
 	defer httpServer.Close()
+	dataImage := "data:image/png;base64," + base64.StdEncoding.EncodeToString(paddedPNG(0))
 
 	for _, imageCount := range []int{3, 6, 9} {
 		content := []any{map[string]any{"type": "text", "text": "multi-reference video"}}
@@ -159,11 +183,13 @@ func TestVolcesProtocolAcceptsThreeSixAndNineReferenceImages(t *testing.T) {
 			if imageCount > 3 && index == imageCount-1 {
 				role = "last_frame"
 			}
+			imageURL := fmt.Sprintf("%s/fixtures/image-%02d.png", httpServer.URL, index%4)
+			if index%2 == 0 {
+				imageURL = dataImage
+			}
 			content = append(content, map[string]any{
 				"type": "image_url", "role": role,
-				"image_url": map[string]any{
-					"url": fmt.Sprintf("%s/fixtures/image-%02d.png", httpServer.URL, index%4),
-				},
+				"image_url": map[string]any{"url": imageURL},
 			})
 		}
 		body, _ := json.Marshal(map[string]any{"model": "seedance-test", "content": content, "seed": imageCount})