Fix PowerShell command failure status

[snoopuppy582]

Summary

• preserve $? before checking LASTEXITCODE in generated PowerShell startup scripts
• return exit code 1 when a PowerShell command fails before setting LASTEXITCODE
• add a Windows regression test for missing-command failures in both powershell and pwsh

Test plan

• pytest src/rez/tests/test_shells.py::TestShells::test_powershell_command_not_found_returncode -q
• pytest src/rez/tests/test_shells.py::TestShells::test_pwsh_lastexitcode_gui -q
• flake8 src/rezplugins/shell/_utils/powershell_base.py src/rez/tests/test_shells.py
• manual execute_shell check: missing command returns 1 for powershell and pwsh; hello_world -q -r 66 still returns 66 for both

Closes #2073

[linux-foundation-easycla]

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• ✅ login: snoopuppy582 / name: snoopuppy582 (72a9d88)

[maxnbk]

@snoopuppy582 was this issue tackled as part of devdays, or independently? Thank you.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.64%. Comparing base (d415b96) to head (72a9d88).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2109      +/-   ##
==========================================
- Coverage   60.65%   60.64%   -0.01%     
==========================================
  Files         164      164              
  Lines       20584    20584              
  Branches     3579     3579              
==========================================
- Hits        12485    12483       -2     
- Misses       7224     7225       +1     
- Partials      875      876       +1     

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[snoopuppy582]

This was independent. I saw #2073, reproduced the PowerShell status issue locally on Windows, and opened the PR with the focused regression test for that case. Thanks for checking.