revert: drop _connect_nio thread-pool executor, restore async _ubridge_send

The _connect_nio thread-pool optimisation (send_batch_sync) targeted
node-start performance, but start_all already runs at concurrency=3
(by design, to avoid overwhelming the host).  It also introduced a
Python 3.13 incompatibility (trsock.setblocking forbidden) that
prevented docker nodes from starting.  Since node-start is not the
target of this branch (project-open link creation is), revert to the
simple per-command async _ubridge_send.

The project-open batch NIO dispatch (create_batch_nios) is unaffected —
it never called _connect_nio (nodes aren't started during open).

tests/compute/docker/test_docker_vm.py

@@ -1440,12 +1440,9 @@ async def test_add_ubridge_connection(vm):
         call.send('bridge create bridge0'),
         call.send("bridge add_nio_tap bridge0 tap-gns3-e0"),
         call.send('docker move_to_ns tap-gns3-e0 42 eth0'),
-        call.send_batch_sync([
-            'bridge add_nio_udp bridge0 4242 127.0.0.1 4343',
-            'bridge start_capture bridge0 "/tmp/capture.pcap"',
-            'bridge start bridge0',
-            'bridge reset_packet_filters bridge0',
-        ]),
+        call.send('bridge add_nio_udp bridge0 4242 127.0.0.1 4343'),
+        call.send('bridge start_capture bridge0 "/tmp/capture.pcap"'),
+        call.send('bridge start bridge0'),
     ]
     assert 'bridge0' in vm._bridges
     # We need to check any_order otherwise mock is confused by asyncio