Better Call routing bug can lead to Cache Deception

Summary

### Summary

Using a CDN that caches (`/**/*.png`, `/**/*.json`, `/**/*.css`, etc...) requests, a cache deception can emerge. This could lead to unauthorized access to user sessions and personal data when cached responses are served to other users.

### Details

The vulnerability occurs in the request processing logic where path sanitization is insufficient. The library splits the path using `config.basePath` but doesn't properly validate the remaining path components. This allows specially crafted requests that appear to be static assets (like `/api/auth/get-session/api/auth/image.png` assuming `config.basePath`=`/api/auth`) to bypass typical CDN cache exclusion rules while actually returning sensitive data.

The problematic code [here](https://github.com/Bekacru/better-call/blob/8b6f13e24fad7f4666a582601517bb3232d4f4af/src/router.ts#L124): ```js const processRequest = async (request: Request) => { const url = new URL(request.url); const path = config?.basePath ? url.pathname.split(config.basePath)[1] : url.pathname; ```

Since this library is largely coupled with `better-auth`, it becomes more clear why this can be dangerous with an example request:

<img width="800" alt="image" src="https://github.com/user-attachments/assets/2ab7c4dd-0700-4f59-863f-79f2b5edbb37" />

### Impact

This is a cache deception vulnerability affecting `better-call` users with CDN caching enabled. which can expose sensitive data.