extension_cli: Disallow slash command extensions (#56864)

This will make CI error when we receive updates or new slash command
extensions.

Release Notes:

- N/A

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Co-authored-by: Marshall Bowers <git@maxdeviant.com>
533e975df0Finn Evers committed on 5/18/2026, 12:59:22 PM· committed by GitHubparent79f998a
3 files changedLine totals unavailable