[change] VpnClient objects should block modifications

Describe the bug
Currently, VpnClient objects allow direct modification of their fields. This is problematic because it can lead to inconsistent state and data integrity issues. The expected behavior is that VpnClient objects should be immutable after creation, and any configuration changes should require re-creating the object instead of modifying existing fields.

Expected behavior
VpnClient objects should not allow direct field modifications. Instead, users should be required to delete the existing VpnClient object and create a new one (by re-applying template) with the desired configuration. This ensures data consistency and makes the object lifecycle management explicit and predictable.

We shall add model validation to disallow any changes to the VpnClient fields post creation.

System Informatioon:

• OS: [e.g. Ubuntu 24.04 LTS]
• Python Version: [e.g. Python 3.11.2]
• Django Version: [e.g. Django 4.2.5]
• Browser and Browser Version (if applicable): [e.g. Chromium v126.0.6478.126]

Start by locating the VpnClient model and its relevant tests, then inspect how field changes are currently validated and persisted. Done means post-creation field modifications are rejected while deleting and re-creating the object remains possible, with tests covering both behaviors.