Flush the C runtime on Windows when redirecting solver output

[yemine0x01]

No description provided.

[romeokienzler]

@yemine0x01 thanks for the Windows portability fix — nicely scoped, and I like that you mirrored the ucrtbase selection in the test so it exercises the real code path on Windows.

What's needed

• CI is still in flight (pytests, dynamic-pytests, CodeQL pending; pre-commit and security-test already green). Just confirm those go green — nothing in the diff looks problematic.
• Per CONTRIBUTING, please add Alban Puech as a reviewer if you haven't yet.

Nothing else jumps out: the change is coherent with the _LIBC.fflush(None) call in redirect_fds, sys is already imported in both files, and there are no new params/deps needing YAML or pyproject.toml updates. I'll leave the merge call to a maintainer.

— 🤖 _automated pre-review; a maintainer will follow up_