[Router] Fleet-wide sampling contract 3/3: splice injection without re-serializing (#39002)

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/main.rs

@@ -24,6 +24,7 @@ mod pd_pool_isolation;
 mod pd_protocol_binding;
 mod radix_tree_routing;
 mod roundrobin_input_ids;
+mod sampling_overrides;
 mod shared_prefill_admission;
 mod sticky_input_ids;
 mod sticky_routing;