Retrieve a customer by the idempotency key used during creation.

This endpoint is useful for:

• Recovery after network failures: If the POST /v1/customers/lightweight call timed out, use this endpoint to check whether the customer was actually created.
• Deduplication checks: Verify whether a customer already exists for a given idempotency key before attempting re-creation.

Returns the same LightweightCustomerResponse as the creation endpoint, including the current KYB status.