ci: migrate controller tests to Pester 6

[lipkau]

Summary

• Pin the controller CI dependency to Pester 6.2.0.
• Require Pester 6.2.0 through 6.x for controller tests.
• Preserve controller logic, fixtures, and workflow behavior.

Refs AtlassianPS/AtlassianPS#12

Validation

• Pester 5.7.1 baseline: 6 passed, 0 failed, 0 skipped, 0 failed blocks, 0 failed containers.
• Pester 6.2.0: 6 passed, 0 failed, 0 skipped, 0 failed blocks, 0 failed containers.
• Invoke-ScriptAnalyzer dependency-updates -Recurse -Severity Warning,Error -EnableExit passed with PSScriptAnalyzer 1.25.0.
• actionlint 1.7.12 passed against all workflows.
• git diff --check passed.
• Independent review found no issues.

Test-count change: none.