Do not fail a node because the engine's own stdout is gone

The log tee wrote through to the real stream unguarded, and the worker
pool tees a returned call's logs there after reading its result and
before handing it back — so a dead stdout, which `fluksio serve` makes
possible by running the engine as a child of the dashboard holding that
pipe, failed the node with its outputs already in hand. The capture half
runs first, so swallowing the write loses nothing.

Also: `flow_events` catches the RuntimeError a peer leaving mid-send
raises, which is a disconnect by another route, and the remote agent no
longer raises out of the task when its subprocess died before it could
be written to — the read below reports that and ends the call.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TXQv6KNyyvY7Z1etYTUUAd

backend/fluksio/api/routes/flows.py

@@ -1026,7 +1026,10 @@ async def flow_events(websocket: WebSocket, token: str = "") -> None:
                     out.append(event)
                 if out:
                     await _send(websocket, out)
-        except WebSocketDisconnect:
+        except (WebSocketDisconnect, RuntimeError):
+            # A peer that goes away mid-send takes the RuntimeError route
+            # ("websocket.send after websocket.close") rather than the clean
+            # disconnect. Either way the socket is gone and the loop is over.
             pass
         finally:
             receiver.cancel()