Refactor Eltex model to use centralized VLAN range expansion utility

- Replaced the internal `_expand_vlan_range` function in the `Eltex` class with the new `expand_vlan_range` utility from `utils.py` for improved code maintainability.
- Added a new configuration file `config.conf` for Eltex devices to facilitate testing.
- Introduced an expected output JSON file `config.expected.json` to validate the parsing of Eltex configurations against expected results.

tests/fixtures/eltex/config.conf

@@ -0,0 +1,55 @@
+! 
+Active-image: flash://system/images/mes3300-669-3R3.ros
+!   Version: 6.6.9.3
+!   Commit: 3a5c2e39
+!   Build: 3 (master)
+!   MD5 Digest: 7bc289cc18be560954bd5cb0afd9b2d5
+!   Date: 22-Sep-2025
+!   Time: 12:38:20
+! Inactive-image: flash://system/images/_image1.bin
+!   Version: 6.6.2
+!   Commit: 3ebc7503
+!   Build: 5 (master)
+!   MD5 Digest: a3f15a788c97c71e07e90d84c0ff3b12
+!   Date: 20-Nov-2023
+!   Time: 16:39:20
+! 
+! Unit    MAC address    Hardware version Serial number 
+! ---- ----------------- ---------------- ------------- 
+!  1   90:54:b7:6b:9d:40     01.01.01      ESG7007778   
+!  2   90:54:b7:6b:bb:80     01.01.01      ESG7007777   
+! 
+! 
+!
+interface TenGigabitEthernet1/0/2
+ shutdown
+ description FREE
+ storm-control broadcast pps 3000
+ storm-control multicast pps 3000
+exit
+!
+interface TenGigabitEthernet1/0/11
+ shutdown
+ description FREE
+ storm-control broadcast pps 3000
+ storm-control multicast pps 3000
+exit
+!
+interface vlan 1700
+ name sw-test_HW
+ ip address 13.36.8.1 255.255.255.0
+exit
+!
+vlan database
+ vlan 114-115,120,130,414,610,999-1000,1701-1703,1705,1801,2001,2011
+ vlan 2021-2022,3157-3158,3333-3334
+exit
+!
+interface vlan 666
+ name test
+exit
+!
+interface vlan 777
+ name test2
+exit
+!

tests/fixtures/eltex/config.expected.json

@@ -0,0 +1,137 @@
+{
+  "system": {
+    "model": "",
+    "serial_number": "ESG7007778",
+    "version": "6.6.9.3"
+  },
+  "interfaces": [
+    {
+      "interface": "TenGigabitEthernet1/0/2",
+      "ip_address": null,
+      "mask": null,
+      "description": "FREE"
+    },
+    {
+      "interface": "TenGigabitEthernet1/0/11",
+      "ip_address": null,
+      "mask": null,
+      "description": "FREE"
+    },
+    {
+      "interface": "vlan 1700",
+      "ip_address": "13.36.8.1",
+      "mask": 24,
+      "description": "sw-test_HW"
+    },
+    {
+      "interface": "vlan 666",
+      "ip_address": null,
+      "mask": null,
+      "description": "test"
+    },  
+    {
+      "interface": "vlan 777",
+      "ip_address": null,
+      "mask": null,
+      "description": "test2"
+    }
+  ],
+  "vlans": [
+    {
+      "vlan_id": 1700,
+      "description": "sw-test_HW"
+    },
+    {
+      "vlan_id": 114,
+      "description": null
+    },
+    {
+      "vlan_id": 115,
+      "description": null
+    },
+    {
+      "vlan_id": 120,
+      "description": null
+    },
+    {
+      "vlan_id": 130,
+      "description": null
+    },
+    {
+      "vlan_id": 414,
+      "description": null
+    },
+    {
+      "vlan_id": 610,
+      "description": null
+    },
+    {
+      "vlan_id": 999,
+      "description": null
+    },
+    {
+      "vlan_id": 1000,
+      "description": null
+    },
+    {
+      "vlan_id": 1701,
+      "description": null
+    },
+    {
+      "vlan_id": 1702,
+      "description": null
+    },
+    {
+      "vlan_id": 1703,
+      "description": null
+    },
+    {
+      "vlan_id": 1705,
+      "description": null
+    },
+    {
+      "vlan_id": 1801,
+      "description": null
+    },
+    {
+      "vlan_id": 2001,
+      "description": null
+    },
+    {
+      "vlan_id": 2011,
+      "description": null
+    },
+    {
+      "vlan_id": 2021,
+      "description": null
+    },
+    {
+      "vlan_id": 2022,
+      "description": null
+    },
+    {
+      "vlan_id": 3157,
+      "description": null
+    },
+    {
+      "vlan_id": 3158,
+      "description": null
+    },
+    {
+      "vlan_id": 3333,
+      "description": null
+    },
+    {
+      "vlan_id": 3334,
+      "description": null
+    },
+    {
+      "vlan_id": 666,
+      "description": "test"
+    },
+    {
+      "vlan_id": 777,
+      "description": "test2"
+    }
+  ]
+}

tests/fixtures/eltex/not_found.conf

@@ -0,0 +1 @@
+node not found