Tests: Publish-PSBuildModule

[HeyItsGilbert]

Add Pester 6 integration tests for Publish-PSBuildModule.

Scenarios to cover:

• Calls Publish-Module with the correct parameters
• Mock the PSRepository to avoid real publishing side effects
• Depends on shared fixture module (see Tests: Create shared fixture module #97)

Target Pester 6.0.0 with classic Should syntax — see notes on #17. Parent issue: #17