Fix contract assertion warning cleanup

[Logiar]

Summary

• replace the previous dead-code workaround with an explicit compile-time trait-object assertion for ContractSurface
• keep EpochApiContract conformance enforced without relying on constructing a throwaway value
• remove the unused-struct warning path while preserving behavior

Test Plan

• cargo test --all-targets --all-features
• RUSTFLAGS=\"-D warnings\" cargo test --all-targets --all-features