[NowinskiK]

Fixes #501

Summary

Makes the unit tests run reliably on Pester 6 (still compatible with Pester 5) and in the VS Code PowerShell host.

Pester 6 compatibility

• Replaced Assert-MockCalled (removed in Pester 6) with Should -Invoke. Calling it made PowerShell autoload the built-in Pester 3.4.0, whose Should then shadowed the real one ('-Not' is not a valid Should operator).
• Test-AdfCode.Tests.ps1: added a default pass-through mock for Get-ChildItem - Pester 6 no longer falls through to the real command when no -ParameterFilter matches.
• !RunAllTests.ps1: unloads Pester < 5.3.3 before importing.

Host independence

• !RunAllTests.ps1 and TestHelper\Get-RootPath no longer use $psEditor.GetEditorContext(), which throws in VS Code when the active tab isn't a file. Both now use $PSScriptRoot. Get-RootPath also returned test\TestHelper instead of test in ConsoleHost.

Module change

• New-AdfPublishOption: missing -FilterFilePath now throws ADFT0026 instead of Write-Error, so it fails regardless of the caller's $ErrorActionPreference (consistent with Publish-AdfV2FromJson not yet failing even using errorActionPreference: stop #472). Previously the test was order-dependent, and callers with default EAP silently got empty filters.
  ⚠️ Behaviour change - worth a changelog entry.

Other

• .gitignore: ignore test/**/~*.json temp files produced by unit tests.
• README: new Running tests locally section (PowerShell 7, modules, Node.js/npm, Az login) and npm prerequisite note for Export-AdfToArmTemplate.
• Publish-AdfV2FromJson-1.Tests.ps1: temporarily disabled "adf2 Should deploy credential1 to ADF service".

Testing

• Affected non-Azure test files pass on both Pester 5.7.1 and 6.2.0.
• PublishOption.Tests.ps1 passes standalone and via the runner.
• Azure-dependent tests not re-run here (no access to the test subscription during this change).

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