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.

tests/api/routes/compute/test_capabilities.py

@@ -29,7 +29,7 @@ pytestmark = pytest.mark.asyncio
 
 
 async def _no_tap_probe():
-    """The tap probe spawns uBridge — stub it out for payload assertions."""
+    """The probes spawn uBridge — stub them out for payload assertions."""
 
     return None
 
@@ -42,6 +42,7 @@ class TestCapabilitiesRoutes:
 
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tc_capabilities", no_probe)
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", _no_tap_probe)
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_iol_tap_support", _no_tap_probe)
         response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
         assert response.status_code == status.HTTP_200_OK
         assert response.json() == {
@@ -67,6 +68,7 @@ class TestCapabilitiesRoutes:
             "disk_size": psutil.disk_usage(get_default_project_directory()).total,
             "ubridge_tc": None,
             "ubridge_tap": None,
+            "ubridge_iol_tap": None,
         }
 
     async def test_get_on_gns3vm(self, app: FastAPI, compute_client: AsyncClient, on_gns3vm, monkeypatch) -> None:
@@ -76,6 +78,7 @@ class TestCapabilitiesRoutes:
 
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tc_capabilities", no_probe)
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", _no_tap_probe)
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_iol_tap_support", _no_tap_probe)
         response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
         assert response.status_code == status.HTTP_200_OK
         assert response.json() == {
@@ -101,6 +104,7 @@ class TestCapabilitiesRoutes:
             "disk_size": psutil.disk_usage(get_default_project_directory()).total,
             "ubridge_tc": None,
             "ubridge_tap": None,
+            "ubridge_iol_tap": None,
         }
 
     async def test_get_reports_ubridge_tc(self, app: FastAPI, compute_client: AsyncClient, monkeypatch) -> None:
@@ -110,6 +114,7 @@ class TestCapabilitiesRoutes:
 
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tc_capabilities", fake_probe)
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", _no_tap_probe)
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_iol_tap_support", _no_tap_probe)
         response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
         assert response.status_code == status.HTTP_200_OK
         assert response.json()["ubridge_tc"] == {
@@ -129,6 +134,22 @@ class TestCapabilitiesRoutes:
             return True
 
         monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", fake_tap_probe)
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_iol_tap_support", _no_tap_probe)
         response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
         assert response.status_code == status.HTTP_200_OK
         assert response.json()["ubridge_tap"] is True
+
+    async def test_get_reports_ubridge_iol_tap(self, app: FastAPI, compute_client: AsyncClient, monkeypatch) -> None:
+        """
+        The IOL-port TAP capability is what makes an IOU node kernel-eligible,
+        so it travels with the other compute capabilities.
+        """
+
+        async def fake_iol_tap_probe():
+            return True
+
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_iol_tap_support", fake_iol_tap_probe)
+        monkeypatch.setattr("gns3server.api.routes.compute.capabilities.probe_tap_support", _no_tap_probe)
+        response = await compute_client.get(app.url_path_for("compute:get_capabilities"))
+        assert response.status_code == status.HTTP_200_OK
+        assert response.json()["ubridge_iol_tap"] is True