fix: get_dhcp_leases raises when no lease source answered (#27)

When Kea, ISC DHCP and the ARP fallback all failed, get_dhcp_leases()
returned [], which a caller cannot tell from a server with no leases. It now
raises a RuntimeError naming each failure, and returns [] only when a source
answered with nothing. An empty ARP table after both lease sources failed
counts as unread too, since ARP rows are no lease data.

Closes #27

napalm_opnsense/opnsense.py

@@ -1316,7 +1316,16 @@ class OPNsenseDriver(
         2. **ISC DHCP** (legacy) — ``POST /api/dhcpv4/leases/searchlease``
            Fields: ``mac``, ``address``, ``hostname``, ``ends``
         3. **ARP table** fallback — ``GET /api/diagnostics/interface/get_arp``
-           Provides IP only, hostname will be empty.
+           Provides IP only, hostname will be empty, ``state`` is ``"arp"``.
+
+        The first source with leases wins. Without any, the ARP table stands
+        in -- also when Kea or ISC answered empty, since a box can hand out
+        addresses through a service neither covers. An empty list is a real
+        answer only: Kea or ISC answered with no leases and the fallback added
+        none. When neither answered and the fallback gives no rows (it failed,
+        or the table is empty), there is no answer to give, and this raises
+        :class:`RuntimeError` naming each failure -- a caller would otherwise
+        take ``[]`` for a server without leases and drop the ones it knows.
 
         Each returned entry contains:
 
@@ -1349,6 +1358,9 @@ class OPNsenseDriver(
                 })
             return result
 
+        failures: list[str] = []
+        answered = False  # a lease source answered, with or without leases
+
         # 1. Kea DHCPv4 plugin
         try:
             data = self._get("/api/kea/leases4/search")
@@ -1356,8 +1368,10 @@ class OPNsenseDriver(
             leases = _parse_kea(rows)
             if leases:
                 return leases
+            answered = True
         except Exception as exc:
             logger.debug("Kea DHCP lease fetch failed: %s", exc)
+            failures.append(f"Kea: {exc}")
 
         # 2. ISC DHCP (legacy)
         try:
@@ -1368,20 +1382,27 @@ class OPNsenseDriver(
             leases = _parse_kea(data.get("rows", []))
             if leases:
                 return leases
+            answered = True
         except Exception as exc:
             logger.debug("ISC DHCP lease fetch failed: %s", exc)
+            failures.append(f"ISC DHCP: {exc}")
 
         # 3. ARP table fallback (IP only, no hostname)
+        fallback: list[dict[str, Any]] = []
         try:
-            arp = self.get_arp_table()
-            return [
+            fallback = [
                 {"mac": e["mac"], "ip": e["ip"], "hostname": "", "lease_end": 0, "state": "arp"}
-                for e in arp
+                for e in self.get_arp_table()
                 if e.get("mac") and e.get("ip")
             ]
+            if not fallback:
+                failures.append("ARP table: no entries")
         except Exception as exc:
             logger.warning("ARP table fallback failed: %s", exc)
-            return []
+            failures.append(f"ARP table: {exc}")
+        if fallback or answered:
+            return fallback
+        raise RuntimeError("No DHCP lease source answered -- " + "; ".join(failures))
 
     def create_dhcp_reservation(self, mac: str, ip: str, hostname: str = "") -> None:
         """Create (or update) a Kea DHCPv4 static reservation (MAC → IP).