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

worker/fluksio_worker/agent.py

@@ -257,7 +257,11 @@ class Agent:
 
         heartbeat = asyncio.create_task(beat())
         try:
-            await loop.run_in_executor(None, worker.send, request)
+            # A subprocess that died before it could be written to is reported
+            # by the read below, which says so and ends the call — rather than
+            # raising here and leaving the engine waiting out its silence.
+            with contextlib.suppress(OSError):
+                await loop.run_in_executor(None, worker.send, request)
             while True:
                 line = await loop.run_in_executor(None, worker.read_line)
                 if not line: