MixedSetter 默认值不生效问题

**StringSetter 的初始值在初始化时并不生效 （任意 setter 不仅限于 StringSetter）**
```
{
...,
setter: {
componentName: 'MixedSetter',
props: {
setters: [
{
componentName: 'StringSetter',
initialValue: '初始值',
},
]
}
}
}
```
**预期行为**
上述配置能成功设置初始值

Start by reproducing the MixedSetter configuration shown in the issue and trace how its nested StringSetter, or any setter, handles initialValue during initialization. The fix is done when the configured initial value is applied successfully for MixedSetter and the behavior is verified for setters beyond StringSetter.

Written by the indexing model from the issue text.