fix: four real defects the fourteen failing tests were pointing at

Closes netork#115.

The suite had been red long enough that it stopped being read. Four of the
fourteen failures were the tests being right.

`interfaces_mixin.py` used `re.match` without importing `re`, so
`get_mac_address_table` raised NameError against any node with a Linux bridge.
The tests never reached that line: they mocked the API call underneath
`_exec_ssh_command`, which takes two positional arguments where the doubles
accepted one, and which base64-wraps the command — so a fixture keyed on
"bridge fdb" appearing in the text matched nothing and the helper returned "".
They mock `_exec_ssh_command` itself now, which is the driver's own seam.

`is_alive` called `_resolve_node()`, which returns early without touching the
API whenever a node was configured through optional_args. A dead connection
reported itself alive. It probes `GET /version` now.

The documented `realm` optional_arg was read into `self._realm` in `__init__`
and then never used. Proxmox authenticates against "<user>@<realm>" and rejects
a bare username, so the option had no effect and callers had to know to type the
realm themselves.

`get_vlans` filtered out entries with no member ports on one return path while
the OVS path returned them, so a configured SDN VNet was visible or invisible
depending on which branch ran. A VNet exists on the node whether or not anything
is attached to it, and netOrk's VLAN discovery reads this.

`get_ipv6_neighbors_table` was simply missing and fell through to NAPALM's stub;
it is implemented against `ip -6 neigh show`, dropping FAILED entries.

The rest were stale tests. The DNS fixture put an FQDN where a search domain
belongs, which made `get_facts` build "pve1.pve1.example.com" and look like a
driver bug. The LLDP fixture was a simplified shape that real `lldpcli show
neighbors summary` does not produce — the parser matches on the ", via: LLDP"
that follows the interface name. And `test_bridge_vlan_show_parsing` covered a
fallback that was replaced by VM-config scanning, asserting an "interfaces" key
this method has never returned; it is now a test of the fallback that exists.

tests/test_sdn.py

@@ -22,19 +22,33 @@ class TestGetVlans:
         result = driver.get_vlans()
         assert "100000" in result
 
-    def test_bridge_vlan_show_parsing(self, driver):
-        # Simulate bridge vlan output
-        bridge_output = (
-            "vmbr0   1\n"
-            "        10\n"
-            "        20\n"
-            "eth0    1\n"
-        )
-        driver._node_api().execute.post.return_value = {"data": bridge_output}
+    def test_membership_derived_from_vm_configs(self, driver):
+        """Without OVS ports, VLAN membership comes from each VM's netN config.
+
+        This replaces a test for a `bridge vlan show` fallback that no longer
+        exists — it also asserted an "interfaces" key this method has never
+        produced, so it could not have passed against any version of the code.
+        """
+        node = driver._node_api()
+        node.qemu.get.return_value = [{"vmid": 100}]
+        node.lxc.get.return_value = []
+        node.qemu.return_value.config.get.return_value = {
+            "net0": "virtio=AA:BB:CC:DD:EE:FF,bridge=vmbr0,tag=10",
+        }
+
         result = driver.get_vlans()
-        # Interface vmbr0 should appear in vlan 1
-        entry = result.get("1", {})
-        assert "vmbr0" in entry.get("interfaces", [])
+        assert "vmbr0" in result["10"]["untagged"]
+
+    def test_configured_vnets_appear_even_without_members(self, driver):
+        """An SDN VNet exists on the node whether or not anything is attached.
+
+        Entries with no member ports used to be filtered out of this one return
+        path while the OVS path returned them, so a configured VLAN was visible
+        or invisible depending on which branch ran.
+        """
+        result = driver.get_vlans()
+        assert result["20"]["name"] == "vnet1"
+        assert result["20"]["untagged"] == []
 
     def test_empty_sdn_returns_dict(self):
         from unittest.mock import patch