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

[MRDGH2821]

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.