Supervisor should send agent_disconnect message on shutdown

Currently the server needs to rely on the online threshold expiring to show a disconnected collector in its actual state.

If we send the agent_disconnect message (which the library might already do) and handle it server side, we can immediately reflect this in its status.

See https://opentelemetry.io/docs/specs/opamp/#agenttoserveragent_disconnect

Start at the Supervisor shutdown path and trace how the server currently determines collector connectivity. Check whether the library already emits the OpAMP agent_disconnect message, then follow the server-side handling of that message. Done means a shutting-down collector is reflected as disconnected immediately instead of waiting for the online threshold to expire.

Written by the indexing model from the issue text.