tests: harden RPC fuzz harness setup

Initialize RPC server state used by direct handler calls,
let fuzz input choose restricted or unrestricted mode,
and skip iterations when the dummy core fails to initialize.

Also update the RPC fuzz harness documentation after removal
of the old RPC payment setup.

src/rpc/core_rpc_server.cpp

@@ -120,9 +120,11 @@ namespace cryptonote
   core_rpc_server::core_rpc_server(
       core& cr
     , nodetool::node_server<cryptonote::t_cryptonote_protocol_handler<cryptonote::core> >& p2p
+    , bool restricted
     )
     : m_core(cr)
     , m_p2p(p2p)
+    , m_restricted(restricted)
     , disable_rpc_ban(false)
   {}
   //------------------------------------------------------------------------------------------------------------------------------