Use a compatible PowerShell fallback for the Windows MXC sandbox - #49019

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## Why

Microsoft Store PowerShell cannot run under MXC. The existing fallback to an unpackaged PowerShell executable only applied to the elevated Windows sandbox.

## What changed

Extend local PowerShell fallback discovery to MXC in `unified_exec`, using the executor's Windows sandbox selection. Keep discovery local and automatic `-NoProfile` insertion specific to the elevated sandbox. Skip this preparation for non-Windows target paths.

## Testing

Extend command-preparation tests to cover MXC and add a Windows integration test that replaces a Store PowerShell path before launch, writes a file successfully, and verifies `windows_mxc` sandbox metadata.

GitOrigin-RevId: 6c581396525a8fec7e1fb538b3e63b0de2ddb926