fix(acceptance): 使用可完整解码的 WebP 样本

替换截断的 WebP 验收夹具，新增全量图片像素解码回归，并保留 6K 外部水化与三参考图归一化测试，避免协议模拟样本缺陷被误判为 Worker 运行故障。\n\n验证：Go 全量测试、go vet、gofmt、git diff --check 通过。

apps/api/internal/acceptanceemulator/server_test.go

@@ -62,11 +62,18 @@ func TestGeminiAndVolcesProtocolEmulation(t *testing.T) {
 	if err != nil {
 		t.Fatalf("get oversized fixture: %v", err)
 	}
-	config, format, err := image.DecodeConfig(fixtureResponse.Body)
+	fixturePayload, readErr := io.ReadAll(fixtureResponse.Body)
 	_ = fixtureResponse.Body.Close()
+	if readErr != nil {
+		t.Fatalf("read oversized fixture: %v", readErr)
+	}
+	config, format, err := image.DecodeConfig(bytes.NewReader(fixturePayload))
 	if err != nil || format != "jpeg" || config.Width != 6144 || config.Height != 2160 {
 		t.Fatalf("oversized fixture format=%s config=%+v err=%v", format, config, err)
 	}
+	if _, format, err = image.Decode(bytes.NewReader(fixturePayload)); err != nil || format != "jpeg" {
+		t.Fatalf("fully decode oversized fixture format=%s err=%v", format, err)
+	}
 
 	content := []any{map[string]any{"type": "text", "text": "video"}}
 	for index, name := range []string{"image-00.png", "image-04.jpg", "image-08.webp"} {
@@ -148,6 +155,22 @@ func TestForcedConversionRejectsImagesOutsideOfficialSeedanceRange(t *testing.T)
 	}
 }
 
+func TestImageFixturesFullyDecode(t *testing.T) {
+	for name, fixture := range buildFixtures() {
+		if !strings.HasPrefix(fixture.ContentType, "image/") {
+			continue
+		}
+		decoded, format, err := image.Decode(bytes.NewReader(fixture.Payload))
+		if err != nil {
+			t.Fatalf("fixture %s does not fully decode: %v", name, err)
+		}
+		bounds := decoded.Bounds()
+		if format == "" || bounds.Dx() <= 0 || bounds.Dy() <= 0 {
+			t.Fatalf("fixture %s format=%q bounds=%v", name, format, bounds)
+		}
+	}
+}
+
 func TestPaddedPNGIsValidAndExact(t *testing.T) {
 	for _, size := range []int{256 << 10, 4 << 20, 8 << 20} {
 		payload := paddedPNG(size)