net, libs: add NAD ref change connectivity helpers

Add shared helpers for NAD live-update tests:

- secondary_iface_addresses(): generate per-IP-family CIDR addresses
  for a secondary interface from a seed and host number.
- ip_family_predicate(): lookup_iface_status predicate that passes
  once the guest agent reports IPs covering all expected IP families.
- update_nad_references(): atomic VM spec patch to update one or more
  secondary network NAD references in a single API call.
- assert_tcp_connectivity() / _poll_tcp_connectivity(): poll TCP
  reachability between two VMs with optional client_bind_dev and
  server_bind_dev to force iperf3 traffic through specific interfaces,
  bypassing ECMP when both secondaries share the same subnet.

Signed-off-by: Asia Khromov <azhivovk@redhat.com>
Assisted-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: azhivovk

tests/network/libs/connectivity.py

@@ -1,5 +1,11 @@
 import ipaddress
 
+from ocp_resources.resource import ResourceEditor
+from timeout_sampler import TimeoutExpiredError, retry
+
+from libs.net.traffic_generator import IPERF_SERVER_PORT, TcpServer, VMTcpClient
+from libs.vm.vm import BaseVirtualMachine
+
 
 def build_ping_command(dst_ip: str, count: int, timeout: int) -> str:
     """
@@ -16,3 +22,52 @@ def build_ping_command(dst_ip: str, count: int, timeout: int) -> str:
     ip = ipaddress.ip_address(address=dst_ip)
     ping_ipv6_flag = " -6" if ip.version == 6 else ""
     return f"ping{ping_ipv6_flag} {dst_ip} -c {count} -w {timeout}"
+
+
+def update_nad_references(vm: BaseVirtualMachine, updates: dict[str, str]) -> None:
+    """Patch the VM spec to update multiple secondary network NAD references in a single atomic call.
+
+    Args:
+        vm: The virtual machine to update.
+        updates: Mapping of interface name to new NAD name.
+    """
+    networks = vm.instance.spec.template.spec.networks
+    for network in networks:
+        if network["name"] in updates:
+            network["multus"].update({"networkName": updates[network["name"]]})
+    ResourceEditor(patches={vm: {"spec": {"template": {"spec": {"networks": networks}}}}}).update()
+
+
+@retry(wait_timeout=60, sleep=5, exceptions_dict={})
+def poll_tcp_connectivity(
+    client_vm: BaseVirtualMachine,
+    server_vm: BaseVirtualMachine,
+    server_ip: str,
+    client_bind_dev: str | None = None,
+    server_bind_dev: str | None = None,
+    expect_connectivity: bool = True,
+) -> bool:
+    """Poll TCP connectivity (or its absence) between two VMs, retrying until the expected state is reached.
+
+    Args:
+        client_vm: VM initiating the TCP connection.
+        server_vm: VM running the iperf3 server.
+        server_ip: IP address the server binds to.
+        client_bind_dev: Guest network device name to force the client out (e.g. "eth1").
+            Bypasses ECMP routing when both secondary interfaces share the same subnet.
+        server_bind_dev: Guest network device name to force the server responses out (e.g. "eth1").
+            Bypasses ECMP routing on the server VM when it has multiple secondary interfaces.
+        expect_connectivity: When True polls until connectivity exists; when False polls until it does not.
+
+    Returns:
+        True when the observed reachability matches expect_connectivity.
+    """
+    try:
+        with TcpServer(vm=server_vm, port=IPERF_SERVER_PORT, bind_ip=server_ip, bind_dev=server_bind_dev):
+            with VMTcpClient(
+                vm=client_vm, server_ip=server_ip, server_port=IPERF_SERVER_PORT, bind_dev=client_bind_dev
+            ):
+                reachable = True
+    except TimeoutExpiredError:
+        reachable = False
+    return reachable if expect_connectivity else not reachable