fix(runner): 修复转存后残留二进制误判

统一结果二进制探测与上传规则，避免将 thinking_bytes 等上游元数据误判为待转存媒体，同时保留显式媒体字段和签名识别。\n\n补充不含原始内容的安全诊断，并兼容对象存储前缀旧字段；修正真实 OSS 验收脚本使用的正式字段。\n\n验证：Go 全量测试、go vet、迁移安全检查、真实 Gemini 上游响应及阿里云 OSS 转存均通过。

apps/api/internal/runner/object_storage.go

@@ -291,7 +291,10 @@ func (a *objectStorageAdapter) objectKey(payload FileUploadPayload) string {
 	now := a.now().UTC()
 	digest := sha256.Sum256(payload.Bytes)
 	extension := uploadFileExtension(payload.ContentType, path.Ext(payload.FileName))
-	prefix := strings.Trim(objectStorageConfigString(a.channel.Config, "objectPrefix"), "/")
+	prefix := strings.Trim(firstNonEmptyString(
+		objectStorageConfigString(a.channel.Config, "objectPrefix"),
+		objectStorageConfigString(a.channel.Config, "objectKeyPrefix"),
+	), "/")
 	parts := make([]string, 0, 7)
 	if prefix != "" {
 		parts = append(parts, prefix)