NGFW-15241: Integrate WAN Balancer weight-based active WAN selection with IPsec VPN and WAN Failover

[AristaKB]

Summary

• Add hook-based communication between WAN Balancer, WAN Failover, and IPsec VPN apps
• Implement weight-based active WAN selection in WAN Failover when WAN Balancer is running
• Initialize IPsec VPN with correct active WAN on startup when WAN Failover is already running
• Fallback to first-active-WAN logic when WAN Balancer is not running

Changes

• HookManager: Added WAN_BALANCER_CHANGE hook for inter-app communication
• WAN Balancer: Trigger hook callbacks on settings changes, start, and stop events to notify WAN Failover
• WAN Failover:
  • Listen to WAN_BALANCER_CHANGE hook and store weight information
  • Update active WAN selection logic to use highest-weight active WAN when WAN Balancer is running
  • Fall back to first-active-WAN when WAN Balancer is stopped
• IPsec VPN: Initialize active WAN from WAN Failover on startup to ensure correct WAN is used

Test Plan

• Test IPsec VPN startup with WAN Failover running - verify correct active WAN is selected
• Test WAN Balancer weight changes - verify active WAN updates accordingly
• Test WAN Balancer stop/start - verify failover switches between weight-based and first-active-WAN logic
• Test with multiple WANs with different weights - verify highest-weight active WAN is selected
• Test failover behavior when active WAN goes down