`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.

Bắt đầu tại điểm vào của lệnh copilot plugin marketplace add và theo dõi cách lỗi marketplace đã tồn tại được xử lý. Chạy lại lệnh với marketplace ponytail, sau đó xác minh rằng marketplace đã được đăng ký sẽ thoát với mã trạng thái 0 và không in ra lỗi.