Completes a bank link enrollment, transitioning it from pending to completed state. The request body must contain state: "completed".

When a bank link is completed, the system exchanges the enrollment credentials with the provider, creates a bank subscription (connection), and discovers the associated bank accounts.

This endpoint is idempotent: completing an already-completed bank link succeeds without change.

The PATCH response echoes the bank link record only. The discovered bankAccounts (with each account's cardIds and any minted cards) are returned by GET /bank-links/{bankLinkId}, not by this PATCH.