Fix Windows PowerShell 5.1 test failures surfaced by reusable CI workflow

[HeyItsGilbert]

Goal

The reusable CI workflow added in #497 (org powershell-ci.yml) is the first CI configuration to ever run the Pester suite under Windows PowerShell 5.1 (the legacy github.yml only tested pwsh/PS7 on windows-latest/macOS-latest/ubuntu-latest). That run failed:

Tests Passed: 720, Failed: 13, Skipped: 8, Inconclusive: 0, NotRun: 0

Run: https://github.com/PowerShellOrg/PSKoans/actions/runs/35290428430/job/105431858030

Failing tests (13 total):

• Invoke-Koan.Tests.ps1: runs the test successfully, produces output with -PassThru, correctly reports test results, reports only expected exception types
• New-KoanRunspace.Tests.ps1: creates a new runspace, names the new runspace, preloads the runspace with PSKoans
• Get-Karma.Tests.ps1 (Context "Behaviour When All Koans Are Completed"): does not divide by zero if all Koans are completed, outputs the result object, indicates completion, indicates number of koans passed, indicates total number of koans, indicates the requested topic

All failures cluster around runspace creation/execution (New-KoanRunspace, Invoke-Koan) and a Get-Karma completion-state context, suggesting a PS5.1-specific incompatibility in how koan test runspaces are created or awaited (New-KoanRunspace.ps1) that cascades into the dependents.

Acceptance criteria

• Root-caused: identify why runspace creation/execution behaves differently under Windows PowerShell 5.1 vs PowerShell 7.
• ci / Run Tests (PowerShell 5.1) is green in Replace legacy PSKoans CI with PowerShellOrg reusable workflow #488's reusable CI workflow.\n- No regressions in the PS7 legs (windows-latest/macOS-latest/ubuntu-latest).\n\nBlocks Replace legacy PSKoans CI with PowerShellOrg reusable workflow #488.