Stabilize pipe process stdin round-trip test

[aibrahim-oai]

What changed

• keep the explicit stdin-close behavior after writing so the child still receives EOF deterministically
• on Windows, stop using python -c for the round-trip assertion and instead run a native cmd.exe pipeline that reads one line from stdin with set /p and echoes it back
• send on Windows so the stdin payload matches the platform-native line ending the shell reader expects

Why this fixes flakiness

The failing branch-local flake was not in spawn_pipe_process itself. The child exited cleanly, but the Windows ARM runner sometimes produced an empty stdout string when the test used Python as the stdin consumer. That makes the test sensitive to Python startup and stdin-close timing rather than the pipe primitive we actually want to validate. Switching the Windows path to a native cmd.exe reader keeps the assertion focused on our pipe behavior: bytes written to stdin should come back on stdout before EOF closes the process. The explicit write removes line-ending ambiguity on Windows.

Scope

• test-only
• no production logic change

[aibrahim-oai]

@codex review this

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Disable cmd AutoRun when asserting exact split output

This test now routes Windows through shell_command, which invokes cmd.exe /C without /D. cmd can execute AutoRun scripts, which may emit extra stdout/stderr and break the exact assert_eq! checks for split-out/split-err, creating environment-dependent flakes. The prior Windows path avoided cmd entirely.

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