Set LASTEXITCODE when Invoke-Pester fails internally

[pathei-kosmos]

Invoke-Pester could overwrite the exit status of an internal error with $null when the run was not created yet, or with 0 when a plugin failed after an otherwise successful run.

This change tracks the process exit code separately from the test failure count so internal errors keep the existing -1 exit code without changing how test failures are reported.

Regression tests cover both failure paths.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@b6eaa72). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3020   +/-   ##
=======================================
  Coverage        ?   91.44%           
=======================================
  Files           ?        2           
  Lines           ?     8354           
  Branches        ?        0           
=======================================
  Hits            ?     7639           
  Misses          ?      715           
  Partials        ?        0           

Flag	Coverage Δ
ps51-windows-2022	90.89% <ø> (?)
ps51-windows-latest	90.89% <ø> (?)
ps7-macos-14	90.30% <ø> (?)
ps7-macos-latest	90.30% <ø> (?)
ps7-ubuntu-2204	90.31% <ø> (?)
ps7-ubuntu-latest	90.31% <ø> (?)
ps7-windows-2022	90.98% <ø> (?)
ps7-windows-latest	90.98% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[nohwnd]

Ty!