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Issue Description

I expect thatproject: true should search fortsconfig.json files in thetsconfigRootDir, even if it's written with a trailing slash.

Reproduction Repository Link

https://github.com/kirkwaiblinger/typescript-eslint-project-true-trailing-pathsep

Repro Steps

1. clone the repo
2. npm i
3. npm run lint

Versions

{"eslint":"9.31.0","typescript":"5.8.3","typescript-eslint":"8.37.0"}

Additional Info

See#11407 and#11398 (comment)

Likely, this doesn't have a huge user impact, because__dirname andimport.meta.dirname don't include the trailing path sep, but we managed to cause bugs for ourselves when programmatically inferringtsconfigRootDir!