feat(ubridge): probe iol_bridge's TAP-port support

Whether an IOU port can terminate on a persistent TAP (iol_bridge
add_nio_tap) is a capability of its own: the tap module and that command
land independently, so a build may have one without the other. Probed on
a scratch IOL bridge with a scratch TAP — one add/delete cycle, no
iol_bridge start needed (attaching a NIO works on a stopped bridge) —
cached by binary identity, with the scratch bridge id above the per-node
id space the fabric locks (513..1024) so a probe cannot collide with a
running node. Any failure, including an old build's "Unknown command",
means unknown: IOU stays on the relay datapath.

Reported through /capabilities as ubridge_iol_tap, next to ubridge_tap.

gns3server/api/routes/compute/capabilities.py

@@ -25,7 +25,7 @@ from fastapi import APIRouter, Request
 
 from gns3server import schemas
 from gns3server.compute import MODULES
-from gns3server.compute.ubridge.tc_probe import probe_tc_capabilities, probe_tap_support
+from gns3server.compute.ubridge.tc_probe import probe_iol_tap_support, probe_tc_capabilities, probe_tap_support
 from gns3server.utils.path import get_default_project_directory
 from gns3server.utils.tc_capabilities import usable_ebpf_modes
 from gns3server.version import __version__
@@ -56,6 +56,10 @@ async def get_capabilities(request: Request) -> dict:
     # datapath. None = unknown (probe failed or uBridge too old).
     ubridge_tap = await probe_tap_support()
 
+    # IOL-port TAP termination (iol_bridge add_nio_tap): the anchor IOU's
+    # Ethernet ports need for the kernel datapath. None = unknown.
+    ubridge_iol_tap = await probe_iol_tap_support()
+
     # record the controller hostname or IP address
     if request.client:
         request.app.state.controller_host = request.client.host
@@ -69,4 +73,5 @@ async def get_capabilities(request: Request) -> dict:
         "node_types": node_types,
         "ubridge_tc": ubridge_tc,
         "ubridge_tap": ubridge_tap,
+        "ubridge_iol_tap": ubridge_iol_tap,
     }