[codex] fix railway token auth preflight

[m-abdelwahab]

Summary

• Treat RAILWAY_TOKEN as valid credentials for the railway up auth preflight.
• Keep user/workspace-token behavior intact by routing the preflight through a new Configs::has_auth_credentials() helper.
• Add focused regression coverage for project-token credentials.

Root cause

The newer railway up first-run auth flow checked get_railway_auth_token() to decide whether to fail fast or start interactive login. That method intentionally excludes RAILWAY_TOKEN, so CI deployments using project tokens exited with Not signed in before the existing project-token deploy path could run.

Fixes #947.

Validation

• cargo test