Fix completion of provider paths when a path returns itself instead of its children

[MartinGC94]

PR Summary

Fixes an issue when trying to complete an item in a path as if it was a container in providers that returns the item itself when calling Get-ChildItem on an item. This would make it complete the same item name again like this: Provider::Container1\Item1\<Tab> -> Provider::Container1\Item1\Item1

PR Context

Fixes #24744

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[iSazonov]

We could use a test provider from related issue and compile it with Add-Type. (There are examples in our tests, one or more is in Add-Type.Tests.ps1)

[MartinGC94]

Test added. I took inspiration from ImportProvider.Tests.ps1 and copied his .cs code.

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