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.

napalm_proxmox/sdn_mixin.py

@@ -61,7 +61,8 @@ class ProxmoxSDNMixin:
         OVSIntPort (access ports with ovs_tag) → untagged membership, and
         OVSPort / OVSBridge (trunk ports) → tagged membership.
 
-        Falls back to ``bridge vlan show`` for classic Linux-bridge nodes.
+        Without OVS ports, VLAN membership is derived from the ``tag=`` values
+        in each VM's and container's ``netN`` config (:meth:`_get_vm_vlan_tags`).
         """
         result: dict[str, _JsonDict] = {}
         node_network = self._get_node_network()
@@ -113,10 +114,12 @@ class ProxmoxSDNMixin:
                 if bridge not in entry["untagged"]:
                     entry["untagged"].append(bridge)
 
-        return {
-            vid: entry for vid, entry in result.items()
-            if entry.get("tagged") or entry.get("untagged")
-        }
+        # Deliberately unfiltered. This used to drop entries with no member
+        # ports, which hid every configured SDN VNet that no VM happened to be
+        # attached to — and contradicted the OVS branch above, which returns
+        # empty-membership VLANs. A VNet exists on the node whether or not
+        # anything currently uses it, and netOrk's VLAN discovery reads this.
+        return result
 
     def _get_vm_vlan_tags(self) -> dict[str, set[str]]:
         """Return ``{vlan_tag: {bridge_names}}`` derived from VM/container net configs.