Restart from the tree no longer hangs for 30 s on disconnect

With a wiresheet open, disconnecting first unsubscribed its components and
waited for the reply, which a restarting device never sends, so the editor
froze until the request timed out. After restart, reboot, quit or hibernate
the session is now closed without asking the device anything; a plain
Disconnect still unsubscribes first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

device_session.py

@@ -60,11 +60,13 @@ class DeviceSession:
                 self.sox_client.update(children, CONFIG | RUNTIME)
         return root_comp
 
-    def disconnect(self):
+    def disconnect(self, unsubscribe=True):
+        """Closes the session. unsubscribe=False skips telling the device first -- for a
+        device going down, which would leave the request to time out."""
         if self.sox_client is None:
             return
         try:
-            if self._current_subs:
+            if unsubscribe and self._current_subs:
                 comps = [self.sox_client.cache_get(cid)
                          for cid in self._current_subs]
                 comps = [c for c in comps if c is not None]