fix(protocol): Seedream 声明式结果 - 裸 b64_json 补全 data URL

声明式解析把 Ark/Seedream 的裸 b64_json 写成 DataURL 时未加 data: 前缀，导致图片任务在上游成功后仍报媒体结果下载失败。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/app/provider_audio.go

@@ -172,7 +172,7 @@ func asyncAudioErrorMessage(state map[string]interface{}) string {
 func decodeProviderDataURL(value string) (string, []byte, error) {
 	header, encoded, ok := strings.Cut(value, ",")
 	if !ok || !strings.HasPrefix(header, "data:") || !strings.HasSuffix(strings.ToLower(header), ";base64") {
-		return "", nil, errors.New("音频 data URL 格式无效")
+		return "", nil, errors.New("data URL 格式无效")
 	}
 	mimeType := strings.TrimSuffix(strings.TrimPrefix(header, "data:"), ";base64")
 	data, err := base64.StdEncoding.DecodeString(encoded)

backend/internal/protocol/manifest.go

@@ -711,6 +711,8 @@ func mediaReferencesFromManifestValue(value any, kind string, ephemeral bool) []
 			if reference.DataURL == "" {
 				reference.DataURL = firstString(typed, "data_url", "b64_json")
 			}
+			// OpenAI / Ark 等渠道常直接返回裸 b64_json；声明式结果下载要求 data URL。
+			reference.DataURL = normalizeManifestInlineDataURL(reference.DataURL, reference.Kind, firstString(typed, "output_format", "mime_type", "mimeType"))
 			if reference.URL != "" || reference.DataURL != "" {
 				result = append(result, reference)
 			}
@@ -719,6 +721,50 @@ func mediaReferencesFromManifestValue(value any, kind string, ephemeral bool) []
 	return result
 }
 
+func normalizeManifestInlineDataURL(value, kind, formatHint string) string {
+	value = strings.TrimSpace(value)
+	if value == "" || strings.HasPrefix(value, "data:") {
+		return value
+	}
+	return "data:" + manifestInlineMediaMIME(kind, formatHint) + ";base64," + value
+}
+
+func manifestInlineMediaMIME(kind, formatHint string) string {
+	hint := strings.ToLower(strings.TrimSpace(formatHint))
+	switch hint {
+	case "image/png", "image/jpeg", "image/webp", "image/gif", "audio/mpeg", "audio/wav", "audio/ogg", "video/mp4", "video/webm":
+		return hint
+	case "image/jpg":
+		return "image/jpeg"
+	case "audio/mp3":
+		return "audio/mpeg"
+	case "png":
+		return "image/png"
+	case "jpeg", "jpg":
+		return "image/jpeg"
+	case "webp":
+		return "image/webp"
+	case "gif":
+		return "image/gif"
+	case "mp3", "mpeg":
+		return "audio/mpeg"
+	case "wav":
+		return "audio/wav"
+	case "mp4":
+		return "video/mp4"
+	case "webm":
+		return "video/webm"
+	}
+	switch strings.ToLower(strings.TrimSpace(kind)) {
+	case "audio":
+		return "audio/mpeg"
+	case "video":
+		return "video/mp4"
+	default:
+		return "image/png"
+	}
+}
+
 var (
 	manifestMDImageRegex   = regexp.MustCompile(`!\[[^\]]*\]\(([^)\s]+)\)`)
 	manifestHTMLVideoRegex = regexp.MustCompile(`(?i)<video[^>]+src=['"]([^'"]+)['"]`)

backend/internal/protocol/official_catalog_test.go

@@ -784,3 +784,27 @@ func TestOfficialOpenAIAudioSpeedDefaultsInvalidAndZeroValues(t *testing.T) {
 		})
 	}
 }
+
+func TestOfficialArkSeedreamParsesB64JSONAsDataURL(t *testing.T) {
+	for _, tc := range []struct {
+		packageName, providerID string
+	}{
+		{"volcengine-ark-seedream.yingce-plugin", "volcengine-ark-image"},
+		{"volcengine-ark-agent-plan-seedream.yingce-plugin", "volcengine-ark-agent-plan-image"},
+	} {
+		t.Run(tc.providerID, func(t *testing.T) {
+			adapter := officialPackageAdapter(t, tc.packageName, tc.providerID)
+			result, err := adapter.ParseCreate(context.Background(), []byte(`{"created":1,"data":[{"b64_json":"aW1hZ2U=","output_format":"jpeg","size":"1824x1024"}]}`))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if result.Result == nil || len(result.Result.Images) != 1 {
+				t.Fatalf("result = %#v", result.Result)
+			}
+			dataURL := result.Result.Images[0].DataURL
+			if dataURL != "data:image/jpeg;base64,aW1hZ2U=" {
+				t.Fatalf("DataURL = %q, want jpeg data URL from b64_json + output_format", dataURL)
+			}
+		})
+	}
+}