Answer the verification step LinkedIn asked for. `code` is the verification code for 2FA and OTP, the phone number for PHONE_REGISTER (with its dialling code in brackets, e.g. `(+33)0612345678`), or the literal `TRY_ANOTHER_WAY` to make LinkedIn offer a different method — the way out of an in-app approval nobody can reach. `status` is `connected` once the sender is live, or `checkpoint_required` with the next `checkpoint` when LinkedIn chained another step. A rejected, expired or unanswerable challenge is a 400 whose `error.code` is one of `checkpoint_code_rejected`, `checkpoint_expired` (the five-minute window closed, so reconnect to start a fresh attempt), `checkpoint_rate_limited`, `checkpoint_restricted`, `checkpoint_no_code` (this type has nothing to type; approve it in the LinkedIn app or on the hosted page), `checkpoint_no_alternative`, `checkpoint_phone_format`, `checkpoint_empty_code`, `checkpoint_none` (nothing is pending), `checkpoint_account_gone` or `checkpoint_not_connected`. When the challenge itself was fine but the attempt could not reach LinkedIn, the 400 carries `proxy_failed` (the sender's proxy did not respond), `provider_timeout` or `rate_limited` instead. A 502 means LinkedIn could not be reached at all: retry.