onGetConfig 支持 deepmerge 的 option

### Summary | 概述

_No response_

### Motivation | 背景

taskConfig 的配置目前已经比较复杂，在做一些 deep merge 操作时会比较麻烦，比如 plugin-rax-compat，期望在 API 层面直接支持该能力

### Usage example | 使用示例

_No response_

### Detailed design | 方案设计

使用方式类似 modifyUserConfig，调用时增加 option

### Additional context | 额外信息

_No response_

Start by locating the onGetConfig implementation and comparing its call path with modifyUserConfig. Review how taskConfig is currently assembled and inspect the plugin-rax-compat use case mentioned in the issue. Done means the API accepts the specified option and supports the intended deep-merge behavior, with coverage for the resulting configuration.

Written by the indexing model from the issue text.