fix: 将 DriveClient 注入 GeminiOAuthService，消除单元测试中的真实 HTTP 调用

FetchGoogleOneTier 原先在方法内部直接创建 DriveClient 实例，
导致单元测试中对 googleapis.com 发起真实 HTTP 请求，在 CI 环境
产生 401 错误。

将 DriveClient 作为依赖注入到 GeminiOAuthService，遵循项目
端口与适配器架构规范：
- 新增 repository/gemini_drive_client.go 作为 Provider
- 注册到 repository Wire ProviderSet
- 测试中使用 mockDriveClient 替代真实调用

backend/internal/service/gemini_oauth_service.go

@@ -54,6 +54,7 @@ type GeminiOAuthService struct {
 	proxyRepo    ProxyRepository
 	oauthClient  GeminiOAuthClient
 	codeAssist   GeminiCliCodeAssistClient
+	driveClient  geminicli.DriveClient
 	cfg          *config.Config
 }
 
@@ -66,6 +67,7 @@ func NewGeminiOAuthService(
 	proxyRepo ProxyRepository,
 	oauthClient GeminiOAuthClient,
 	codeAssist GeminiCliCodeAssistClient,
+	driveClient geminicli.DriveClient,
 	cfg *config.Config,
 ) *GeminiOAuthService {
 	return &GeminiOAuthService{
@@ -73,6 +75,7 @@ func NewGeminiOAuthService(
 		proxyRepo:    proxyRepo,
 		oauthClient:  oauthClient,
 		codeAssist:   codeAssist,
+		driveClient:  driveClient,
 		cfg:          cfg,
 	}
 }
@@ -362,9 +365,8 @@ func (s *GeminiOAuthService) FetchGoogleOneTier(ctx context.Context, accessToken
 
 	// Use Drive API to infer tier from storage quota (requires drive.readonly scope)
 	logger.LegacyPrintf("service.gemini_oauth", "[GeminiOAuth] Calling Drive API for storage quota...")
-	driveClient := geminicli.NewDriveClient()
 
-	storageInfo, err := driveClient.GetStorageQuota(ctx, accessToken, proxyURL)
+	storageInfo, err := s.driveClient.GetStorageQuota(ctx, accessToken, proxyURL)
 	if err != nil {
 		// Check if it's a 403 (scope not granted)
 		if strings.Contains(err.Error(), "status 403") {