test(gemini): 添加 Drive API 和 OAuth 服务单元测试

- 新增 drive_client_test.go：Drive API 客户端单元测试
- 新增 gemini_oauth_service_test.go：OAuth 服务单元测试
- 重构 account_handler.go：改进 RefreshTier API 实现
- 优化 drive_client.go：增强错误处理和重试逻辑
- 完善 repository 和 service 层：支持批量 tier 刷新
- 更新迁移文件编号：017 -> 024（避免冲突）

backend/migrations/024_add_gemini_tier_id.sql

@@ -0,0 +1,30 @@
+-- +goose Up
+-- +goose StatementBegin
+-- 为 Gemini Code Assist OAuth 账号添加默认 tier_id
+-- 包括显式标记为 code_assist 的账号，以及 legacy 账号（oauth_type 为空但 project_id 存在）
+UPDATE accounts
+SET credentials = jsonb_set(
+    credentials,
+    '{tier_id}',
+    '"LEGACY"',
+    true
+)
+WHERE platform = 'gemini'
+  AND type = 'oauth'
+  AND jsonb_typeof(credentials) = 'object'
+  AND credentials->>'tier_id' IS NULL
+  AND (
+    credentials->>'oauth_type' = 'code_assist'
+    OR (credentials->>'oauth_type' IS NULL AND credentials->>'project_id' IS NOT NULL)
+  );
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+-- 回滚：删除 tier_id 字段
+UPDATE accounts
+SET credentials = credentials - 'tier_id'
+WHERE platform = 'gemini'
+  AND type = 'oauth'
+  AND credentials ? 'tier_id';
+-- +goose StatementEnd