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/features/providers/components/provider-tabs/model-test-request.ts

@@ -1,5 +1,9 @@
 const DEFAULT_MODEL_TEST_MESSAGE = 'Hello! This is a test message.'
 
+/** Pool-managed provider runs concurrent checks; single-key provider does not. */
+export const POOL_TEST_CONCURRENCY = 5
+export const SINGLE_TEST_CONCURRENCY = 1
+
 export function buildDefaultModelTestRequestBody(modelName: string): string {
   return JSON.stringify({
     model: modelName,
@@ -15,14 +19,23 @@ export function buildDefaultModelTestRequestBody(modelName: string): string {
   }, null, 2)
 }
 
-export function parseModelTestRequestBodyDraft(
+export function buildDefaultModelTestRequestHeaders(): string {
+  return JSON.stringify({}, null, 2)
+}
+
+function parseModelTestJsonObjectDraft(
   draft: string,
+  options: {
+    emptyValue: Record<string, unknown> | null
+    emptyError: string | null
+    invalidTypeError: string
+  },
 ): { value: Record<string, unknown> | null; error: string | null } {
   const normalized = draft.trim()
   if (!normalized) {
     return {
-      value: null,
-      error: '测试请求体不能为空',
+      value: options.emptyValue,
+      error: options.emptyError,
     }
   }
 
@@ -31,7 +44,7 @@ export function parseModelTestRequestBodyDraft(
     if (!parsed || typeof parsed !== 'object' || Array.isArray(parsed)) {
       return {
         value: null,
-        error: '测试请求体必须是 JSON 对象',
+        error: options.invalidTypeError,
       }
     }
     return {
@@ -45,3 +58,23 @@ export function parseModelTestRequestBodyDraft(
     }
   }
 }
+
+export function parseModelTestRequestBodyDraft(
+  draft: string,
+): { value: Record<string, unknown> | null; error: string | null } {
+  return parseModelTestJsonObjectDraft(draft, {
+    emptyValue: null,
+    emptyError: '测试请求体不能为空',
+    invalidTypeError: '测试请求体必须是 JSON 对象',
+  })
+}
+
+export function parseModelTestRequestHeadersDraft(
+  draft: string,
+): { value: Record<string, unknown> | null; error: string | null } {
+  return parseModelTestJsonObjectDraft(draft, {
+    emptyValue: {},
+    emptyError: null,
+    invalidTypeError: '测试请求头必须是 JSON 对象',
+  })
+}