Fix Windows external bearer refresh test (#16366)

## Why

https://github.com/openai/codex/pull/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.

https://github.com/openai/codex/pull/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.

codex-rs/core/tests/suite/client.rs

@@ -190,7 +190,7 @@ mv tokens.next tokens.txt
             let script_path = tempdir.path().join("print-token.ps1");
             std::fs::write(
                 &script_path,
-                r#"$lines = Get-Content -Path tokens.txt
+                r#"$lines = @(Get-Content -Path tokens.txt)
 if ($lines.Count -eq 0) { exit 1 }
 Write-Output $lines[0]
 $lines | Select-Object -Skip 1 | Set-Content -Path tokens.txt