[Code quality] simple the URL build logic in AdminMockFilter.

- [ ] I have searched the [issues](https://github.com/apache/dubbo-spi-extensions/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [REAMDE](https://github.com/apache/dubbo-admin/blob/dubbo-spi-extensions/README.md) of this repository and believe that this is not a duplicate.

### Description
The logic of building URL in AdminMockFilter is too much complex. simple the building logic.
https://github.com/apache/dubbo-spi-extensions/blob/9c5d9dde494b44b3363ebcb8d33e8aa45382d141/dubbo-mock-extensions/dubbo-mock-admin/src/main/java/org/apache/dubbo/mock/filter/AdminMockFilter.java#L98-L116

Start with dubbo-mock-extensions/dubbo-mock-admin/src/main/java/org/apache/dubbo/mock/filter/AdminMockFilter.java, focusing on lines 98-116 and the surrounding URL-building logic. Trace how the current URL is assembled, then simplify that logic while preserving its existing behavior; no test file is specified in the issue.

Written by the indexing model from the issue text.