`copilot plugin marketplace add` should silently succeed if marketplace is already registered

Describe the feature or problem you'd like to solve

copilot plugin marketplace add currently fails when the marketplace is already registered.
Example:

copilot plugin marketplace add https://github.com/DietrichGebert/ponytail 
Failed to add marketplace: Error: Request plugins.marketplaces.add failed with message: Marketplace "ponytail" already registered

Proposed solution

If the target marketplace is already registered, treat it as a no-op and exit successfully (status code 0), ideally without printing an error.

Optional: print a non-error informational message (e.g., "Marketplace already registered") but still return 0.

Example prompts or workflows

• Re-running bootstrap/setup scripts across machines
• CI/devcontainer initialization where marketplace registration may already exist
• User reruns the same command and expects no failure for existing state

Expected Behaviour:

• Command is idempotent.
• "already registered" condition does not produce an error exit.
• Exit code is 0 for already-registered marketplaces.

Additional context

This issue report text was generated using GitHub Copilot.

Commencez au point d’entrée de la commande copilot plugin marketplace add et suivez la manière dont l’erreur liée à un marketplace existant est gérée. Réexécutez ensuite la commande avec le marketplace ponytail, puis vérifiez qu’un marketplace déjà enregistré se termine avec le code d’état 0 et n’affiche aucune erreur.