fix(images): 优先转换编辑图片为 multipart 二进制

OpenAI 编辑即使声明支持 URL，也必须在 multipart 提交前转存为二进制文件。

apps/api/internal/runner/request_assets.go

@@ -159,9 +159,11 @@ func (s *Service) hydrateProviderRequestAssetString(ctx context.Context, value s
 	if raw == "" || !imageInputFieldNeedsHydration(path) {
 		return value, nil
 	}
-	style, ok := requestAssetCapabilityHydrationForMedia("image", candidate, raw, "")
-	if !ok && openAIImageEditRequiresMultipartBytes(candidate) {
+	style, ok := requestAssetHydrateUnsupported, false
+	if openAIImageEditRequiresMultipartBytes(candidate) {
 		style, ok = requestAssetHydrateDataURL, true
+	} else {
+		style, ok = requestAssetCapabilityHydrationForMedia("image", candidate, raw, "")
 	}
 	if !ok {
 		return value, nil
@@ -320,12 +322,12 @@ func requestAssetHydrationForField(path []string, asset store.RequestAsset, cand
 		return requestAssetHydrateDataURL
 	}
 	if requestAssetMediaKindForHydration(path, asset) == "image" {
-		if style, ok := requestAssetCapabilityHydrationForMedia("image", candidate, asset.URL, asset.StorageProvider); ok {
-			return style
-		}
 		if openAIImageEditRequiresMultipartBytes(candidate) {
 			return requestAssetHydrateDataURL
 		}
+		if style, ok := requestAssetCapabilityHydrationForMedia("image", candidate, asset.URL, asset.StorageProvider); ok {
+			return style
+		}
 	}
 	if mediaURLFieldNeedsHydration(path) {
 		if style := configuredRequestAssetMediaURLHydration(candidate, requestAssetMediaURLKind(path)); style != "" {