fix: use COMSPEC in Windows unicode shell test

[bolinfest]

Why

Windows Bazel shell tests launch PowerShell with a curated environment, so PATHEXT may be absent. The existing unicode_output test invokes bare cmd, which can fail before the test exercises UTF-8 child-process output.

What

• Use $env:COMSPEC /c echo naïve_café in the Windows branch of unicode_output.
• Preserve the external child-process path instead of switching the test to a PowerShell builtin.

Verification

• cargo test -p codex-core unicode_output