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/tests/flow/test_logs.py

@@ -122,6 +122,25 @@ def test_printing_outside_a_node_still_reaches_the_real_stream(capsys):
     assert "server talking" in capsys.readouterr().out
 
 
+def test_a_dead_real_stream_does_not_fail_the_node_being_teed():
+    """`fluksio serve` runs the engine as a child of the dashboard."""
+
+    class Broken:
+        def write(self, text: str) -> int:
+            raise BrokenPipeError(32, "Broken pipe")
+
+        def flush(self) -> None:
+            raise BrokenPipeError(32, "Broken pipe")
+
+    collected: list[str] = []
+    tee = logs._Tee(Broken())
+    with logs.capture(collected.append):
+        assert tee.write("still teed") == len("still teed")
+        tee.flush()
+
+    assert collected == ["still teed"]
+
+
 def test_installing_twice_does_not_stack_tees():
     logs.install()
     once = sys.stdout