Migrate test suite from Pester 3/4 to Pester 5

[HeyItsGilbert]

What

• Update all tests in /Tests to Pester 5 syntax
• Fix Should assertions that use legacy positional syntax
• Add -Tag attributes to Describe blocks for filtering
• Ensure tests run cleanly under Invoke-Pester v5

Why

Pester 5 is the current standard. CI will run on PS 7.x runners where Pester 5 is the default.

Notes

This may surface real bugs — document any found as separate issues rather than fixing inline.