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.

(cherry picked from commit 0fa1e99)

(cherry picked from commit b69c014)

…frame body

(cherry picked from commit c258816)