[BUG] -BufferInputStream转字符流读取HttpServletRequest的body会有中文边界乱码问题，都是utf-8编码

### Steps to Reproduce

### Expected Behavior

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### Servicecomb Version

_No response_

### Additional Context

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Start by reproducing the reported HttpServletRequest body read with Chinese UTF-8 characters that cross an input-buffer boundary. Trace the BufferInputStream-to-character-stream path and identify the relevant tests or request-handling entry point. Done means the same UTF-8 request body is read without garbled characters, with a regression test covering the boundary case.

Written by the indexing model from the issue text.