fix: double-wrapped return values caused by Promise.{reject,resolve}

[kripod]

Reasoning 💡

Both Promise.reject and Promise.resolve return a Promise on their own. They are meant to be called inside non-async functions and serve as an escape hatch when native async/await syntax isn’t available.

The code of NextAuth is transpiled, though, so all the async functions already return a Promise by default. Thus, Promise.reject and Promise.resolve aren’t necessary anymore, and they may cause issues like #602.

Checklist 🧢

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• [ ] Documentation
• [ ] Tests
• Ready to be merged
  

Affected issues 🎟

Possibly fixes #602

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[balazsorban44]

I encourage you to open this against the adapters repository, so our migration process can be quicker.

You can read https://github.com/nextauthjs/adapters/issues/37#issuecomment-826413849 for our plans.