[BUG] DefaultBlockExceptionHandler treats all BlockException as FlowException

## Issue Description

Type: *bug report*

### Describe what happened
The DefaultBlockExceptionHandler treat all BlockException as FlowException, even if it is a DegradeException. This is not good.

### Describe what you expected to happen
It will set appropriate http status code and message accroding to the type of BlockException.

### How to reproduce it (as minimally and precisely as possible)

1. try to create a degrade exception
2. it will get 429(too many request) and ”Blocked by Sentinel (flow limiting)"
3.

### Tell us your environment

### Anything else we need to know?

Start by locating DefaultBlockExceptionHandler and tracing how it converts BlockException into an HTTP response. Reproduce a DegradeException using the issue's steps, then verify that each BlockException subtype receives an appropriate HTTP status code and message rather than the flow-limiting response.

Written by the indexing model from the issue text.