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
+!