Cherry picked #5286

[neacsu]

This change is 

[tommyv1987]

Testing Completion Report: Feature/Mix_Tun_Const

Testing Completion Date: 08/01/2025

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Testing Steps Performed:

1. Review File: common/network-defaults/src/constants.rs

   • Confirmed updated mixnet_vpn constants were added.

2. Review File: service-providers/ip-packet-router/src/constants.rs

   • Checked replacement of legacy TUN_* constants with new mixnet_vpn constants.
   • Validated alignment of routing traffic configurations.

3. Review File: service-providers/ip-packet-router/src/ip_packet_router.rs

   • Ensured new nym_network_defaults::constants::mixnet_vpn constants replaced old references.
   • Verified TunDeviceConfig consistency.

4. Review File: service-providers/ip-packet-router/src/util/generate_new_ip.rs

   • Confirmed substitution of TUN_DEVICE_* constants with NYM_TUN_DEVICE_* constants.
   • Tested functionality for generating random IPs within subnet.

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Validation:

• Test Passed
• Issues Found (Please specify below)

Attachments (if applicable):

N/A

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