Bluetooth: Mesh: Proxy feature is broken in Zephyr 4.0.0

[PavelVPV]

Describe the bug
After #78914 the Mesh Proxy feature is broken. It is not possible anymore to connect to the proxy node. This change can also result in mem fault: https://discord.com/channels/720317445772017664/733036850226266133/1311536117740474389

To Reproduce

1. Check out Zephyr 4.0.0
2. Build tests/bluetooth/mesh_shell
3. Start PB-GATT provisioning bearer by typing mesh prov pb-gatt on in shell
4. Use nRF Mesh mobile app for iOS or Android to provision and configure the device
5. Observe that configuration phase fails. The mobile app can't receive Composition Data of the device.

Expected behavior
Configuration works fine over mobile app.

Impact
The Mehs Proxy feature is broken in Zephyr 4.0.0

Environment (please complete the following information):

• Zephyr 4.0.0

[LingaoM]

Currently there is no bbsim for proxy ?!

[PavelVPV]

Currently there is no bbsim for proxy ?!

Those we have test advertisement interval, advertised subnets, etc., but not data transfer between proxy client (mobile phone) and proxy server.

[github-actions]

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