Cilium Feature Proposal

Is your proposed feature related to a problem?
When running cilium-agent with load balancer control plane inside a cluster (e.g., on 3 nodes), performing a rollout can lead to a temporary loss of all BGP announcements while the pods are being restarted sequentially.

Describe the feature you'd like
A readiness probe for the cilium-agent that checks the state of BGP session(s). This would prevent pod transitions during rollouts until BGP sessions are properly established, ensuring continuous service availability.

Describe your proposed solution
Implement BGP session state monitoring in the cilium-agent's readiness probe when there are matching ciliumbgppeeringpolicies. This should:

1. Monitor the BGP control plane state
2. Be configurable via Helm when bgpControlPlane is enabled
3. Include configuration options to specify whether the probe should:
   • Wait for all configured BGP sessions to reach "Established" state
   • OR require at least one session to be "Established"