[FEATURE] Add macOS PowerShell Core testing to CI

[cocallaw]

Feature Description

Add macos-latest to the PowerShell Core test matrix in .github/workflows/ci.yml.

Problem Statement

CI currently validates the module on:

• Ubuntu with PowerShell Core
• Windows with PowerShell Core
• Windows with Windows PowerShell 5.1 Desktop

macOS with PowerShell Core is not covered, leaving platform-specific path, filesystem, encoding, and PowerShell behavior unverified.

Proposed Solution

Extend the existing test-pwsh matrix from:

os: [ubuntu-latest, windows-latest]

to include macos-latest, reusing the current PowerShell edition/version checks and Pester 5.x test steps.

Acceptance Criteria

• macos-latest is included in the PowerShell Core CI matrix.
• The job verifies PSEdition is Core and PowerShell major version is 6 or later.
• Pester remains constrained to version 5.x.
• The full Tests/AzRetirementMonitor.Tests.ps1 suite passes on macOS without live Azure authentication.
• Existing Ubuntu, Windows PowerShell Core, and Windows PowerShell 5.1 jobs remain unchanged and passing.

Benefits

• Improves existing functionality
• Improves cross-platform coverage
• Detects macOS-specific compatibility regressions before release

[cocallaw]

Resolved with PR #81