Two sources of noise in the logs when port scanners connect to the
stream port:

1. Unrecognised pre-auth data was logged at warning level. Since this
   is expected from port scanners and other non-RabbitMQ clients, add a
   specific `{unknown, _}` clause in `handle_frame_pre_auth/4` that
   logs at debug level instead. The existing warning-level catch-all
   remains for any other unexpected pre-auth commands.

   Also add a specific `{unknown, _}` clause in
   `handle_frame_post_auth/4` with a clearer log message, and extract
   `send_close_and_increment/2` to eliminate the resulting duplication.

2. When incoming data does not contain a complete frame (e.g. a port
   scanner sends fewer bytes than a full frame header), no commands are
   parsed and `connection_step` remains unchanged. The pre-`open`
   state handlers treated this as an invalid transition and logged a
   warning. Add a guard in each of the five pre-`open` state handlers
   (`tcp_connected`, `peer_properties_exchanged`, `authenticating`,
   `tuning`, `tuned`) that returns `keep_state` when
   `NextConnectionStep` equals the current state, waiting for more
   data. The existing negotiation timeout will still close connections
   that make no progress.