fix(smtp_relay): use correct async-for bus subscription in probe listener

bus.subscribe() is sync and returns an async iterator, not a coroutine.
Awaiting it caused an immediate crash at startup; bus.next_message() does
not exist either. Rewrote _run_smtp_probe_listener to use the standard
pattern: sub = bus.subscribe(...) / async with sub / async for event in sub.

decnet/orchestrator/worker.py

@@ -483,30 +483,22 @@ async def _run_smtp_probe_listener(
     probe_limit times — if not, forward via the master's real internet
     connection and store a probe_relay bounty with the result.
     """
-    bus = None
     try:
         bus = get_bus(client_name="orchestrator-probe")
         await bus.connect()
-        await bus.subscribe(_topics.smtp("probe.pending"))
+        sub = bus.subscribe(_topics.smtp("probe.pending"))
+        async with sub:
+            async for event in sub:
+                if shutdown.is_set():
+                    break
+                try:
+                    await _handle_probe_pending(repo, event.payload)
+                except Exception as exc:  # noqa: BLE001
+                    logger.warning("smtp probe listener: handle error: %s", exc)
+    except asyncio.CancelledError:
+        raise
     except Exception as exc:  # noqa: BLE001
         logger.warning("smtp probe listener: bus unavailable: %s", exc)
-        return
-
-    try:
-        while not shutdown.is_set():
-            try:
-                msg = await asyncio.wait_for(bus.next_message(), timeout=5.0)
-            except asyncio.TimeoutError:
-                continue
-            except Exception as exc:  # noqa: BLE001
-                logger.debug("smtp probe listener: recv error: %s", exc)
-                continue
-            if msg is None:
-                continue
-            try:
-                await _handle_probe_pending(repo, msg.get("payload") or msg)
-            except Exception as exc:  # noqa: BLE001
-                logger.warning("smtp probe listener: handle error: %s", exc)
     finally:
         with contextlib.suppress(Exception):
             await bus.close()