Merges the accounts in source_account_ids into the account in the path. This is irreversible, like deleting an account: each source account is deleted, its contacts, opportunities, tasks, tags and domains move to the target, and its live sequences are stopped. Call preview_account_merge first.

When live sequences would be stopped the request is rejected with 409 sequence_stop_confirmation_required unless confirm_sequence_stop is true. If a merge fails part-way, 409 merge_failed says which steps already committed; retrying the same request resumes it. Retrying a merge that already completed succeeds with zero new changes and a positive source_accounts_already_merged count. source_account_ids in the response echo the ids as resolved at request time, so a retry that names an already-deleted source by its AccountICP id echoes that id rather than the account id.