TuningManager treated any open non-health connection as activity, so a
client that kept its socket open held the daemon in busy mode and kept
background scaling up. RequestHandler now counts executing non-health
requests with an RAII guard, SocketServer wires the counter into
DaemonStats, and the idle/busy decisions read it.
Tests: request_handler_status_unary, socket_server_teardown, and two slow
real-service tests (tuning_registry_activity, socket_proxy_request_activity).
Signed-off-by: trvon <me@trevon.dev>
