Fix Windows external bearer refresh test

[bolinfest]

Why

#16287 introduced a change to codex-rs/login/src/auth/auth_tests.rs that uses a PowerShell helper to read the next token from tokens.txt and rewrite the remainder back to disk. On Windows, Get-Content can return a scalar when the file has only one remaining line, so $lines[0] reads the first character instead of the full token. That breaks the external bearer refresh test once the token list is nearly exhausted.

#16288 introduced similar changes to codex-rs/core/src/models_manager/manager_tests.rs and codex-rs/core/tests/suite/client.rs.

These went unnoticed because the failures showed up when the test was run via Cargo on Windows, but not in our Bazel harness. Figuring out that Cargo-vs-Bazel delta will happen in a follow-up PR.

Verification

On my Windows machine, I verified cargo test passes when run in codex-rs/login and codex-rs/core. Once this PR is merged, I will keep an eye on https://github.com/openai/codex/actions/workflows/rust-ci-full.yml to verify it goes green.

What changed

• Wrap Get-Content -Path tokens.txt in @(...) so the script always gets array semantics before counting, indexing, and rewriting the remaining lines.