fix(core): route.extensions should work

[SoonIter]

Summary

1. fix: route.extensions should work

in @rspress/core RouteService, the default value is ['md','mdx', ...]

and in @rspress/plugin-auto-nav-sidebar, the default value is ['.md','.mdx', ...]

what a shameful disaster !!!

2. test: add more unit test for route and sidebar core functions

Related Issue

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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