Add error handling for PSGallery repository registration

[Copilot]

Addresses feedback on #4066 regarding Register-PSRepository -Default failing when the repository is already registered.

Changes

• Check if PSGallery exists before attempting registration to prevent duplicate registration errors
• Maintain error visibility for actual registration failures by only suppressing errors on the existence check

# Before
Register-PSRepository -Default

# After
if (-not (Get-PSRepository -Name 'PSGallery' -ErrorAction SilentlyContinue)) {
    Register-PSRepository -Default
}

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