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_ovs_and_arp.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import pytest
+from unittest.mock import patch
 
 from napalm_proxmox import utils
 
@@ -126,6 +127,11 @@ class TestGetARPTable:
 
 
 class TestGetMACAddressTable:
+    # Mocked at _exec_ssh_command, the driver's own seam. Mocking the API call
+    # underneath it broke twice over: that helper passes two positional
+    # arguments where these doubles accepted one, and it base64-wraps the
+    # command, so a fixture keyed on "bridge fdb" appearing in the text never
+    # matched.
     BRIDGE_FDB = (
         "aa:bb:cc:dd:ee:01 dev eth0 vlan 10 master vmbr0 permanent\n"
         "cc:dd:ee:ff:00:11 dev eth0 vlan 20 master vmbr0\n"
@@ -142,10 +148,8 @@ class TestGetMACAddressTable:
                 return self.BRIDGE_FDB
             return ""
 
-        driver._node_api().execute.post.side_effect = lambda command: {
-            "data": _exec(command)
-        }
-        result = driver.get_mac_address_table()
+        with patch.object(driver, "_exec_ssh_command", side_effect=_exec):
+            result = driver.get_mac_address_table()
         macs = {e["mac"] for e in result}
         assert "aa:bb:cc:dd:ee:01" in macs
 
@@ -155,9 +159,7 @@ class TestGetMACAddressTable:
                 return self.BRIDGE_FDB
             return ""
 
-        driver._node_api().execute.post.side_effect = lambda command: {
-            "data": _exec(command)
-        }
-        result = driver.get_mac_address_table()
+        with patch.object(driver, "_exec_ssh_command", side_effect=_exec):
+            result = driver.get_mac_address_table()
         static_entries = [e for e in result if e["mac"] == "aa:bb:cc:dd:ee:01"]
         assert static_entries[0]["static"] is True