fix: preserve command output on Windows

[MobiusL00p]

Summary

• remove the Windows PowerShell wrapper in run_cmd_subprocess so shell commands keep the default Windows shell=True behavior
• add a regression test covering the PowerShell-parent path for commands containing commas
• keep the fix scoped to the Windows run_cmd output regression without changing the pagerank PR

Verification

• python -m pytest -q tests/basic/test_run_cmd.py
• python -c "from aider.run_cmd import run_cmd; code, out = run_cmd('echo Hello, World!'); print('EXIT=', code); print('OUT=', repr(out))"
• python -c "from aider.run_cmd import run_cmd_subprocess; import aider.run_cmd as rc; rc.platform.system=lambda:'Windows'; rc.get_windows_parent_process_name=lambda:'powershell.exe'; code, out = run_cmd_subprocess('echo Hello, World!'); print('EXIT=', code); print('OUT=', repr(out))"