Windows stdin detector has no test coverage (same class of gap as #651)

Summary

stdin_detect.rs has three separate per-platform implementations (linux, macos, windows), and the Windows one has no test coverage at all.

Context

This came out of #651, where the macOS detector had a wrong constant (TH_STATE_WAITING defined as 2, which is TH_STATE_STOPPED) that silently disabled macOS stdin forwarding entirely. It went unnoticed because no CI job executed that code path.

PR #656 closes that for Unix by adding an un-gated stdin-forwarding cohort, now confirmed passing on ubuntu and macos. Windows is still uncovered, for two reasons:

1. test_stdin_forwarding_single_line / _multiple_lines drive head -n1, which stock Windows does not provide.
2. The Windows build is a separate job (windows-build-test) that does not share the matrix job's steps. It runs a hand-maintained list of targeted test names under Run targeted Windows validation tests.

So the same class of bug as #651 could exist in the Windows detector today and CI would be green.

Suggestion

The straightforward version is a Windows-appropriate equivalent of the two stdin-forwarding tests, using a shell built-in or a PowerShell one-liner instead of head -n1, then adding those names to the existing targeted-test list.

Worth considering as an alternative: make the stdin tests portable by having the fixture spawn a small Rust helper that reads one line from stdin, rather than depending on a platform command. That would let one test cover all three detectors instead of maintaining per-platform variants.

I am not implementing either, since choosing between them is a testing-strategy call and I cannot verify Windows behavior from this host.

Not urgent

Filing for tracking rather than as a live bug: there is no evidence the Windows detector is currently broken. The point is that nothing would tell us if it were.

--- — Jcode agent (automated triage), on behalf of @1jehuang

Start with stdin_detect.rs and the existing test_stdin_forwarding_single_line and test_stdin_forwarding_multiple_lines tests. Inspect the windows-build-test job and its targeted-test list, then choose a Windows-appropriate test approach. Done means Windows CI exercises the detector and the relevant stdin-forwarding tests pass.