test: add comprehensive unit tests for provider, middleware, and sync service

- Add TESTING.md documentation explaining unit test conventions and integration testing setup
- Add miniredis and sqlite dependencies to go.mod for in-memory testing
- Add provider_handler_test.go ensuring Vertex providers default google_location to "global"
- Add request_id_test.go verifying request ID generation and header preservation
- Add sync_test.go validating Redis snapshot writes and routing key generation
- Update README.md with testing section referencing new documentation

README.md

@@ -95,6 +95,10 @@ cd ez-api
 
 脚本会拉起 `docker-compose.integration.yml` 中的服务，运行带 `integration` tag 的 Go 测试，并在完成后清理容器和卷。
 
+## 测试
+
+- 单元测试：`go test ./...`（测试文件与源码同目录，更多约定见 `TESTING.md`）
+
 ## 日志
 
 - 业务代码统一使用标准库 `log/slog`（`logger.Info("msg", "k", v)` 风格）。