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/driver.py

@@ -141,9 +141,18 @@ class ProxmoxDriver(
             # The openssh backend tunnels all kwargs through to
             # openssh_wrapper.CommandBaseSession, which does not
             # accept password/verify_ssl/token params.
+            # Proxmox authenticates against "<user>@<realm>" and rejects a bare
+            # username outright. A caller who typed a realm keeps it; one who
+            # did not gets self._realm, which is what the documented `realm`
+            # optional_arg is for -- it was read in __init__ and then never
+            # used, so the option had no effect and a bare username failed.
+            user = self.username or ""
+            if user and "@" not in user:
+                user = f"{user}@{self._realm}"
+
             kwargs: _JsonDict = {
                 "host": self.hostname,
-                "user": self.username,
+                "user": user,
                 "password": self.password,
                 "port": self._port,
                 "verify_ssl": self._verify_ssl,
@@ -210,11 +219,17 @@ class ProxmoxDriver(
             self._ssh_client = None
 
     def is_alive(self) -> _JsonDict:
-        """Return connection liveness."""
+        """Return connection liveness.
+
+        Probes ``GET /version``, the cheapest endpoint that proves the session
+        still authenticates. It used to call ``_resolve_node()``, which returns
+        early without touching the API whenever a node was configured via
+        optional_args — so a dead connection reported itself alive.
+        """
         alive = False
         if self._api:
             try:
-                self._resolve_node()
+                self._api.version.get()
                 alive = True
             except Exception:
                 pass