String object, len is of type unsigned int, however indexOf() and lastIndexOf() return int

### Description

String Object:

len is of type unsigned int.
method that apply to position into the String or its size are expecting an unsigned int, this is the case with:
charAt()
substring()
remove()
reserve()

Oddly, indexOf() and lastIndexOf() are returning int and not unsigned int

### Current behavior

Oddly, indexOf() and lastIndexOf() are returning int and not unsigned int

### Expected behavior

indexOf() and lastIndexOf() ought to return unsigned int, to avoid unnecessary casting from int to unsigned int

### Additional information
It's frequent to pass the result of indexOf() to substring()

Thanks

在 ArduinoCore-API 的 String 实现中查找 indexOf() 和 lastIndexOf() 的声明与实现，然后将它们的返回类型与 len、charAt()、substring()、remove() 和 reserve() 进行比较。确认这两个方法都一致地使用预期的 unsigned 类型，并且将它们的结果传递给 substring() 的调用方不再需要 cast。

由索引模型根据 Issue 内容生成。