grpc: apply the config selector only after the LB policy is updated (#9442) Per [gRFC A31](https://github.com/grpc/proposal/blob/master/A31-xds-timeout-support-and-config-selector.md#implementation-for-xds-routeaction-support:~:text=When%20new%20clusters,INTERNAL%20status%20code.), the channel must update its LB policy before applying a new config selector, and the LB policy must synchronously provide a new picker before its update method returns. This ordering guarantees that a config selector never routes an RPC to a cluster unknown to the current picker. This PR updates the channel to apply the config selector only after `updateClientConnState` returns. Because `updateClientConnState` blocks until the LB policy finishes processing the state update—and `xds_cluster_manager` produces its new picker before returning—the new picker is guaranteed to be in place first. The config selector is then applied while holding `cc.mu`, and the update is skipped if the channel closed or entered idle mode while the lock was released. This preserves two invariants: - The selector reset in `Close()` remains the final write. - A stale update cannot overwrite the selector installed by a new resolver after the channel exits idle mode. Consistent with existing LB policy updates, this relies on the (undocumented) guarantee that resolvers do not invoke `UpdateState` concurrently, as overlapping calls could still apply config selectors out of order. RELEASE NOTES: * xds: Fix a race where RPCs could fail with `UNAVAILABLE: unknown cluster selected for RPC` right after an xDS route configuration update started sending traffic to a new cluster.
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