fix(e2e): headless TkAgg crash + readiness false-positive on forwards

Run 10536 reached pytest but every connect was reset: the simulator died at
import with 'Tkinter is needed' — lib/gui.py calls matplotlib.use("TkAgg")
unconditionally (an explicit use() overrides MPLBACKEND=Agg), and the port
probe passed instantly because it was accepting the localhost forwarders, not
the nodes.

- run_e2e.sh: patch meshtasticator lib/gui.py TkAgg->Agg before boot;
  probe readiness on the upstream host (docker daemon) when forwards are
  active; docker pull meshtastic/meshtasticd up front so the pull is off the
  simulator's node-boot path.
- e2e tests: cancel leftover loop tasks (watchdog) at loop teardown.

plugin/tests/e2e/test_meshtasticator_e2e.py

@@ -143,6 +143,16 @@ def loop():
     try:
         yield _loop
     finally:
+        # Cancel leftover tasks (e.g. a watchdog started by a failed connect
+        # whose disconnect teardown never ran) before closing the loop.
+        async def _shutdown():
+            for task in asyncio.all_tasks(_loop):
+                task.cancel()
+
+        try:
+            asyncio.run_coroutine_threadsafe(_shutdown(), _loop).result(timeout=5)
+        except Exception:
+            pass
         _loop.call_soon_threadsafe(_loop.stop)
         thread.join(timeout=5)