fix(exec): default Windows commands to PowerShell and allow shell parameter
Single-line commands on Windows were routed through cmd.exe
(asyncio.create_subprocess_shell) while multi-line commands used
PowerShell. This caused cross-drive cd failures, missing $VAR expansion,
and inconsistent behavior depending on whether a command contained a
newline. The shell parameter was also rejected on Windows.
- Route all Windows commands through PowerShell by default so single-line
and multi-line commands share the same shell semantics.
- Allow the shell parameter on Windows: accepts powershell, pwsh, or cmd.
- cmd.exe remains reachable via shell="cmd" as an explicit escape hatch.
- Update tests to cover the new default and shell-parameter paths.
Fixes#4544
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dajiaohuangandXubin Ren
33b1c6f601