Upgrade test infrastructure to Pester 5.9.x

[lipkau]

Summary

• install Pester 5.9.0 and bound lint, unit, and integration execution at 5.9.999
• run style tests explicitly with the same bounds and retain analyzer coverage
• replace private-command discovery through module.Invoke with InModuleScope
• bound parallel integration workers to the accepted Pester range

Why

Pester 5.9.0 is the reproducible clean-install baseline. The 5.9.999 maximum accepts compatible 5.9.x patches without allowing an unreviewed future major version to change test behavior.

This applies the lessons from AtlassianPS/JiraAgilePS#43.

Validation

• Invoke-Build -Task Lint, Build, Test: 1,661 core tests passed, 1 skipped; all helper, build, help, project, and style suites passed
• cold-cache dependency setup completed successfully
• Windows PowerShell 5.1, Windows PowerShell 7, Ubuntu, and macOS CI are green (run 32741223108)
• Cloud and Server integration are green on the rebased head (run 32743234609)
• a focused Cloud rerun executed the complete 252-test suite with zero failures (run 32744135515)
• Groups.Integration.Tests.ps1 executed successful cases on both Cloud and Server

[lipkau]

Superseded by #709, which retains the Pester 5.9 changes and adds the broader dependency updater work.