Fix CoreBPE::new type inference failure

[fallintoplace]

Summary

• remove the unused NSE type parameter from CoreBPE::new
• add a Rust regression test that exercises the public constructor through a normal call site

Why

CoreBPE::new exposed an unconstrained generic parameter that never appeared in the arguments or return type. That made normal downstream calls fail type inference with E0283, even though the crate publishes an rlib and the constructor is part of the public Rust API.

Fixes #557.

Validation

• cargo test --manifest-path Cargo.toml
• cargo check --manifest-path /tmp/tiktoken-downstream/Cargo.toml