[Router] Fleet-wide sampling contract 1/3: the config surface (#39000)

Co-authored-by: Kangyan Zhou <kangyan.zhou@radixark.ai>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

experimental/sgl-router/tests/proxy/failover.rs

@@ -46,6 +46,7 @@ async fn failover_when_one_worker_dies() {
             affinity: None,
             fused: None,
             eligibility: None,
+            sampling_overrides: Default::default(),
         },
         discovery: DiscoveryBackend::StaticUrls(StaticUrlsDiscoveryConfig {
             urls: vec![w1.url.clone(), w2.url.clone(), w3.url.clone()],