Credential provider with custom sign in page and a hostname with `signin` word gets replaced by `callback`

Environment

  System:
    OS: macOS 15.3.2
    CPU: (8) arm64 Apple M1 Pro
    Memory: 155.50 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.13.1 - ~/.nvm/versions/node/v22.13.1/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v22.13.1/bin/yarn
    npm: 10.9.2 - ~/.nvm/versions/node/v22.13.1/bin/npm
    pnpm: 10.1.0 - ~/.nvm/versions/node/v22.13.1/bin/pnpm
  Browsers:
    Chrome: 138.0.7204.169
    Safari: 18.3.1
  npmPackages:
    @auth/unstorage-adapter: ^2.0.0 => 2.10.0 
    next: ^14.2.28 => 14.2.28 
    next-auth: 5.0.0-beta.22 => 5.0.0-beta.22 
    react: ^18.2.0 => 18.3.

Reproduction URL

https://github.com/rachirib/next-auth-example-signin

Describe the issue

When a credentials provider is setup with a custom login page and its hostname that has the word signin, the authentication workflow gets broken because the word is replaced by callback.

We found out about this because we have a vercel environment and I created a branch with signin and the entire authentication system was not functioning correctly.

This piece of code it might be related
https://github.com/nextauthjs/next-auth/blob/39dd3b92de194c1a835f2d87631f4deb9d9fdf65/packages/next-auth/src/lib/actions.ts#L65-L67

How to reproduce

https://github.com/rachirib/next-auth-example-signin?tab=readme-ov-file#reproduction-steps

Expected behavior

I'm not sure the intention of this replacement code,

https://github.com/nextauthjs/next-auth/blob/39dd3b92de194c1a835f2d87631f4deb9d9fdf65/packages/next-auth/src/lib/actions.ts#L65-L67

but it might be that the hostname should not be replaced so authentication call can flow in the right domain.

Read packages/next-auth/src/lib/actions.ts around lines 65-67 and inspect how the sign-in hostname is transformed. Run the reproduction steps in the linked next-auth-example-signin repository; the issue is done when a hostname containing signin is preserved and the credentials authentication flow works on the custom sign-in page.