[client] Add support for retain existing AllowedIPs when no alternative paths are available

[gamerslouis]

We are using Netbird to manage our WireGuard network (without relay nodes). Our topology is relatively stable and does not change frequently.

We have analyzed potential network instability issues that may occur when either the ICE connection drops or the management API becomes temporarily unavailable. As noted in this comment, in such cases, network routes (e.g., 10.0.0.0/24) are removed from AllowedIPs, effectively cutting off connectivity.

In our scenario, we prefer:
• When ICE fails or the management service is temporarily down, the existing network connectivity should remain unaffected. The system should not proactively remove AllowedIPs, especially when no alternative paths are available.
• Even if the management service remains operational, the temporary unavailability of routes during ICE reconnection is still unacceptable, as it causes unnecessary and avoidable disruptions.

We propose adding an option to enable a failsafe routing mode, where:
• Route manager only updates AllowedIPs when a valid, reachable path is available.
• If no valid path is detected, the current AllowedIPs are kept unchanged.

This behavior would help prevent unnecessary network disconnections caused by transient ICE or management issues.

Example environment variable:

NB_ROUTE_STICKY_ON_FAILURE=true

Issue ticket number and link

Stack

Checklist

• Is it a bug fix
• Is a typo/documentation fix
• Is a feature enhancement
• It is a refactor
• Created tests that fail without the change (if possible)
• Extended the README / documentation, if necessary

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[gamerslouis]

#4228 might broke our changes.

Currently, ICE disconnection does not remove the endpoint, so we only retain the routes.
If #4228 is merged, we should also disable endpoint removal.