PR Summary

Fixes #28225.

When a native command fails to start on Windows and it is the only command in the pipeline, PowerShell retries with ShellExecute. That retry turns off the stream redirection but leaves StandardOutputEncoding / StandardErrorEncoding set, so Process.Start throws before ShellExecute is tried:

Set-Content notpe.exe 'not an executable'
$x = & .\notpe.exe
# Program 'notpe.exe' failed to run: StandardOutputEncoding is only supported when standard output is redirected.
& .\notpe.exe 2>$null
# Program 'notpe.exe' failed to run: StandardErrorEncoding is only supported when standard error is redirected.

This change clears both encodings together with the redirection flags. With it, both forms report the real start error, the same as & .\notpe.exe | Out-Null.

PR Context

The encodings were added for redirected native output, but the ShellExecute fallback was not updated to clear them, so any captured native command that needs the fallback fails with an unrelated InvalidOperationException instead.

The new test (Windows only) runs a non-PE .exe with stdout captured and with stderr redirected, and checks that the error's inner exception is the Win32Exception from starting the process. It fails without the fix.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge. If this PR is a work in progress, please open this as a Draft Pull Request and mark it as Ready to Review when it is ready to merge.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed