Harden PSDesiredStateConfiguration setup in Pester tests

[Copilot]

Several Pester suites handled PSDesiredStateConfiguration inconsistently: some always attempted installation, while others dereferenced .Version on a potentially missing module. This makes test bootstrap brittle in environments where the module is absent or repository access is unavailable.

• Normalize module detection

  • Guard PSDesiredStateConfiguration installation behind an explicit availability check instead of assuming the module is already present.

• Preserve version-specific behavior where required

  • Keep the existing 2.0.7 requirement in the suites that already depend on it, but check for that version safely before installing.

• Remove unconditional installs

  • Update suites that previously installed PSDesiredStateConfiguration on every run to install only when the module is not already available.

• Files updated

  • tests/GitDsc/GitDsc.tests.ps1
  • tests/Microsoft.DotNet.Dsc/Microsoft.DotNet.Dsc.Tests.ps1
  • tests/Microsoft.Windows.Developer/Microsoft.Windows.Developer.Tests.ps1
  • tests/Microsoft.VSCode.Dsc/Microsoft.VSCode.Dsc.Tests.ps1

if ($null -eq (Get-Module -Name PSDesiredStateConfiguration -ListAvailable | Where-Object Version -eq '2.0.7')) {
    Install-Module -Name PSDesiredStateConfiguration -Force -SkipPublisherCheck -RequiredVersion '2.0.7'
}