feat(provider): 模型测试支持自定义请求头，优化对话框布局与并发策略

- 前后端新增 request_headers 字段，测试时可自定义额外请求头
- ModelTestDialog 拆分为请求头/请求体并排双栏布局，增加格式化与重置按钮
- 区分 Pool 托管(并发5)和单 Key Provider(并发1)的测试并发数
- JsonImportInput 新增 multiple prop 支持单文件模式
- KeyFormDialog Service Account 输入改用 JsonImportInput，支持拖拽导入

frontend/src/composables/useModelTest.ts

@@ -15,6 +15,7 @@ export interface StartTestParams {
   apiFormat?: string
   endpointId?: string
   message?: string
+  requestHeaders?: Record<string, unknown>
   requestBody?: Record<string, unknown>
   concurrency?: number
   onSuccess?: (result: TestModelFailoverResponse) => void
@@ -139,6 +140,7 @@ export function useModelTest(options: UseModelTestOptions) {
         api_format: params.apiFormat,
         endpoint_id: params.endpointId,
         ...(normalizedMessage ? { message: normalizedMessage } : {}),
+        ...(params.requestHeaders ? { request_headers: params.requestHeaders } : {}),
         ...(params.requestBody ? { request_body: params.requestBody } : {}),
         request_id: reqId,
         concurrency: params.concurrency,