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.go

@@ -712,10 +712,14 @@ func buildFixtures() map[string]fixture {
 		_ = jpeg.Encode(&encoded, imageValue, &jpeg.Options{Quality: 88})
 		fixtures[fmt.Sprintf("image-%02d.jpg", index+4)] = fixture{ContentType: "image/jpeg", Payload: encoded.Bytes()}
 	}
-	webpPayload, _ := base64.StdEncoding.DecodeString("UklGRiIAAABXRUJQVlA4IBYAAAAwAQCdASoBAAEALmk0mk0iIiIiIgBoSygABc6zbAAA")
-	for index := 0; index < 4; index++ {
-		payload := append([]byte(nil), webpPayload...)
-		payload = append(payload, byte(index))
+	webpFixtures := []string{
+		"524946465a00000057454250565038204e000000f003009d012a400040003e9148a04c25a42322220800b012096900d3ca8000103b93c116da67710000feeea63fff80dd7c5b4cbfff7381ff7381ff7381fc6d4326178e9af219737c3e2990000000",
+		"5249464650000000574542505650382044000000d003009d012a400040003e9148a04c25a42322220800b012096900760000206ea6a00af10b720000feeed3dfffc5b9fb75d21ffff8b43ad0eb43447bcc63798400000000",
+		"52494646500000005745425056503820440000009003009d012a400040003e9148a04c25a42322220800b0120969000010685716c42e300a2000fef558bffffb9c0fffd9c0fffd9c0fe3afff5ea96b93a3f9696c1c000000",
+		"524946464e0000005745425056503820420000009003009d012a400040003e9148a04c25a42322220800b0120969000010375350057885b90000fef4351ffffba7afffda7afffda7afe60fff239de951800000000000",
+	}
+	for index, encoded := range webpFixtures {
+		payload, _ := hex.DecodeString(encoded)
 		fixtures[fmt.Sprintf("image-%02d.webp", index+8)] = fixture{ContentType: "image/webp", Payload: payload}
 	}
 	for index := 0; index < 4; index++ {