Migrate PowerShell tests to Pester 6

[MSBrett]

Summary

Migrates the PowerShell test suite to Pester 6 after PR #2203 restores the reviewed PR #2163 tree and pins CI back to Pester 5.7.1.

This PR intentionally makes the Pester 6 move explicit instead of relying on unreviewed compatibility shims or post-approval test edits.

Requirements traceability matrix

Requirement	Implementation	Verification
Update CI to Pester 6	Changed the PR validation workflow module cache from Pester 5.7.1 to Pester 6.0.0.	Ran lint and unit tests under Pester 6.0.0.
Remove legacy mock assertions	Migrated remaining Assert-MockCalled usage in PowerShell tests to Should -Invoke.	Confirmed no Assert-MockCalled remains under src/powershell/Tests.
Avoid compatibility shims	Did not add global Pester imports, Assert-MockCalled wrappers, or shared test-suite shims.	Confirmed no Pester global import or Assert-MockCalled shim remains under src/powershell/Tests.
Handle Pester 6 discovery behavior	Added explicit empty data handling for data-driven docs link tests and moved New-Directory setup to BeforeDiscovery.	Pester 6 discovery and execution completed successfully for lint/unit CI.
Validate touched integration assertion	Ran the integration test path under Pester 6.0.0.	The changed mocked long-running CostExports test passed: Should wait 60s when throttled.

Validation

• GitHub Actions Pester 6.0.0 lint: Tests Passed: 2982, Failed: 0
• GitHub Actions Pester 6.0.0 unit: Tests Passed: 1892, Failed: 0, Skipped: 4
• Local Pester 6.0.0 integration run attempted via Start-PesterTest -Type Integration
  • Changed integration assertion passed: Context Long-running unit tests / Should wait 60s when throttled
  • Full integration suite did not pass in this local Azure context due broader integration-suite issues:
    • CostExports live create/update tests failed because the generated storage account has key-based authentication disabled (BadRequest)
    • Hubs.Tests.ps1 failed discovery because Get-FinOpsHubRequiredResourceProvider is not visible from Pester 6 BeforeDiscovery
    • Workbook Resource Graph query execution/performance checks failed/inconclusive with BadRequest
  • Test resource group ftk-integration-tests was left with 0 resources after the run

Stacked on #2203 so this review only covers the explicit Pester 6 migration.