DeviceManager marks a device "busy" as soon as an Appium/WDA session is
connected, which is unrelated to whether a task is currently executing on
it. The Host Agent's local console displayed this raw status, making
connected-but-idle devices look permanently busy. Cross-reference the
device id against AgentStatusTracker's current_assignment (already
tracked via mark_assignment_started/finished) to show "connected" unless
a task is actually running on that device.