enb_proxy: split conn_info() into mme_conn_info() and proxy_info()
The old conn_info() conflated two distinct concerns: the MME SCTP connection info stored in enb_registry (aid, saddr, sport) and the broader operational state used for introspection (handler pid, enb connection info, etc.). Mixing them forced enb_registry to hold a handler pid it has no business knowing about, and required rest_server to extract that pid just to reach s1ap_proxy for E-RAB listing.
Split into two distinct types:
* mme_conn_info() - pure MME SCTP connection info (aid, saddr, sport), stored in the enb_registry and signalled via notify_mme_comm_up/2. The `mme_` prefix is dropped from field names as the type name provides the context.
* proxy_info() - richer operational snapshot (handler, enb_handle, enb_conn_info, mme_conn_info, genb_id_str, mme_info), returned by fetch_info/1 for introspection/debugging purposes.
Additionally:
* Add fetch_erab_list/1 to enb_proxy, delegating internally to s1ap_proxy:fetch_erab_list/1 via the cached handler pid. This allows the rest_server to obtain a list of E-RAB without having to obtain pid of the s1ap_proxy and interact with it.
* Remove separate enb_aid/mme_aid/mme_saddr/mme_sport state fields; enb_aid is now read directly from enb_conn_info, and the MME fields are grouped in mme_conn_info.
