Tests: Initialize-PSBuild

[HeyItsGilbert]

Add Pester 5 unit tests for Initialize-PSBuild.

Scenarios to cover:

• Environment variables and build properties are set correctly
• Path resolution works on both PS 5.1 and PS 7 (see INIT task throws an exception in Windows PowerShell #53)
• Mock Set-BuildEnvironment to avoid filesystem side effects

No fixture module needed. Parent issue: #17