smartcontract: rename multicast group subscribe processor for clarity

[martinsander00]

The process_subscribe_multicastgroup / subscribe_user_to_multicastgroup functions handle both subscribe and unsubscribe operations, but the names only suggest subscribe. This was confusing when reasoning about #3494 which added an unsubscribe-only code path.

Rename the internal functions to process_update_multicastgroup_subscription / update_user_multicastgroup_subscription and simplify the status validation guard to use a positive is_subscribe check instead of double-negated !is_unsubscribe_only.