## What happened?

enable_weighted_failover is meant to retry a failed request inside its model group before falling back to a different group. When the model group also has deployment affinity enabled (session_affinity or another DeploymentAffinityCheck-based check), the retry can lose to a stale affinity pin and fall through to cross-group fallback even though a healthy sibling deployment exists.

Picture a model group with two deployments, A and B, weighted failover on, and session affinity on. A session's first request lands on A and pins there. A then fails. Weighted failover correctly works out that B is healthy and not on cooldown, so it should retry on B. But before the retry actually picks a deployment, the affinity callback runs first, sees the session is still pinned to A, and narrows the candidate list down to just A. Only after that does the exclusion filter drop A, leaving nothing, so the retry errors out and weighted failover gives up. The caller then falls through to the cross-group fallback, or fails outright if none is configured, instead of landing on the healthy B that was right there.

This defeats the purpose of weighted failover for any model group that also …