fix(provider): 修正媒体请求转换与上游错误透传

按上游协议能力延迟处理媒体资源：OpenAI 兼容平台默认使用 multipart，显式配置后才发送 JSON URL；Gemini 官方协议使用 Files API，兼容协议使用内嵌 Base64，并同步覆盖相关媒体客户端。\n\n安全的上游 400/422 原始错误会作为下游 message 返回，同时保留结构化诊断信息和历史任务兼容。\n\n验证：API 全量无缓存测试、go vet、pnpm lint、pnpm test、pnpm build、pnpm openapi、git diff --check。

apps/api/internal/httpapi/easyai_compat_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/publicerror"
 	"github.com/easyai/easyai-ai-gateway/apps/api/internal/store"
 )
 
@@ -191,6 +192,34 @@ func TestEasyAITaskResultResponseMovesImageDataURLToB64JSON(t *testing.T) {
 	}
 }
 
+func TestEasyAITaskResultResponseForwardsSafeUpstreamParameterMessage(t *testing.T) {
+	raw := "Duplicate parameter: 'image'. Use image[]=<value> for multiple values."
+	legacy := publicerror.Error{
+		Code:       "upstream_invalid_request",
+		Message:    "The upstream service rejected the request parameters.",
+		Category:   "upstream",
+		Source:     "upstream",
+		HTTPStatus: http.StatusBadRequest,
+		Version:    "v1",
+	}
+	response := easyAITaskResultResponse(store.GatewayTask{
+		ID:           "image-error-1",
+		Kind:         "images.edits",
+		Status:       "failed",
+		ErrorCode:    "http_400",
+		ErrorMessage: raw,
+		PublicError:  &legacy,
+	})
+	standard, ok := response["error"].(publicerror.Error)
+	if !ok {
+		t.Fatalf("unexpected EasyAI error payload: %+v", response)
+	}
+	upstream, _ := standard.Details["upstreamError"].(map[string]any)
+	if response["message"] != raw || standard.Message != raw || upstream["message"] != raw {
+		t.Fatalf("EasyAI result did not forward safe upstream diagnostics: %+v", response)
+	}
+}
+
 func TestEasyAITaskResultResponsePreservesAudioDataURLAsContent(t *testing.T) {
 	base64Payload := "SUQzBAAAAAAAI1RTU0UAAAAPAAADTGF2ZjYwLjMuMTAwAAAAAAAAAAAAAAD/"
 	task := store.GatewayTask{