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Issue Description

I expect that project: true should search for tsconfig.json files in the tsconfigRootDir, 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 and import.meta.dirname don't include the trailing path sep, but we managed to cause bugs for ourselves when programmatically inferring tsconfigRootDir!