One shared id character check, and a self-address from the environment

Bridge id validation dropped its own pattern and now uses the shared non-empty alphanumeric-dash-underscore check that raises the same error. A new helper derives a cloud session's own reply address from its session id variable, accepting only ids prefixed session_ or cse_ that pass that check, and it backs both the self-address set and the "no reply address" reasons when sending a message. The helper returns nothing outside remote sessions.