The iLink API signals failures through either `ret` or `errcode`.
`_poll_once` already checked both, but `_send_text` and `_send_media_file`
only checked `errcode`. When the API returned `ret != 0` with
`errcode == 0`, the send appeared successful but the message was never
delivered, causing the "still losing messages" issue.
- Add `_check_response_error` helper that validates both fields
- Use it in `_send_text` and `_send_media_file`
- Add debug log after successful text send for observability
- Add test for nonzero ret with zero errcode
Refs: previous inbound fix (suppress -> explicit try/except)
e9f4a868a8