fix: tenant_access_token response is flat, not wrapped in data key

The /auth/v3/tenant_access_token/internal endpoint, like the OAuth token
endpoints, returns code, msg, tenant_access_token, and expire at the top
level of the JSON response — NOT nested under a 'data' key.

FeishuAPIResponse<TenantAccessTokenData> decoded successfully but with
data=nil (extra fields silently ignored by JSONDecoder). The guard then
threw FeishuError.apiError(code: 0, message: 'ok') — appearing to succeed
but always failing. This blocked the subsequent OAuth token exchange call.

- Added code/msg fields to TenantAccessTokenData
- tenantAccessToken() now decodes TenantAccessTokenData directly and
  checks data.code != 0 instead of going through FeishuAPIResponse

Co-Authored-By: Claude <noreply@anthropic.com>

Sources/Services/Feishu/FeishuAuthService.swift

@@ -102,14 +102,13 @@ final class FeishuAuthService {
         var request = URLRequest(url: url)
         request.httpMethod = "POST"
         request.setValue("application/json; charset=utf-8", forHTTPHeaderField: "Content-Type")
-        request.httpBody = try JSONEncoder().encode([
-            "app_id": appId,
-            "app_secret": appSecret
-        ])
+        let body: [String: String] = ["app_id": appId, "app_secret": appSecret]
+        request.httpBody = try JSONEncoder().encode(body)
 
-        let response: FeishuAPIResponse<TenantAccessTokenData> = try await send(request)
-        guard let data = response.data else {
-            throw FeishuError.apiError(code: response.code, message: response.msg)
+        // This endpoint returns code/msg/tenant_access_token/expire all at top level
+        let data: TenantAccessTokenData = try await send(request)
+        guard data.code == 0 else {
+            throw FeishuError.apiError(code: data.code, message: data.msg)
         }
 
         cachedTenantToken = data.tenantAccessToken

Sources/Services/Feishu/FeishuModels.swift

@@ -40,10 +40,14 @@ struct OAuthTokenData: Decodable {
 }
 
 struct TenantAccessTokenData: Decodable {
+    let code: Int
+    let msg: String
     let tenantAccessToken: String
     let expire: Int
 
     enum CodingKeys: String, CodingKey {
+        case code
+        case msg
         case tenantAccessToken = "tenant_access_token"
         case expire
     }