fix: explicitly close WebSocket with code 1000 when loops end

When any of the read/write/heartbeat loops ends, we were cancelling
remaining tasks but not explicitly closing the WebSocket. This caused
the connection to be dropped with 1006 abnormal closure instead of
a clean 1000 close. The frontend then reconnected, creating a loop.

apps/api/src/api/terminal.py

@@ -100,6 +100,12 @@ async def terminal_websocket(
         # Cancel remaining tasks
         for task in pending:
             task.cancel()
+        
+        # Close WebSocket cleanly to avoid 1006 abnormal closure
+        try:
+            await websocket.close(code=1000)
+        except Exception:
+            pass
 
     except Exception as exc:
         logger.error("Terminal session error for instance %s: %s", instance_id, str(exc), exc_info=True)