Enable Tokenizers with Byte Tokens

[lapp0]

Fixes #1038

Problem

Qwen directly uses bytes as tokens for "incomplete unicode". We don't handle type(token) == byte in fsm.py.

For "incomplete unicode" handling with llama and gpt2 style BPE tokenizers, fsm.regex.reduced_vocabulary() looks for a string prefix indicating it's incomplete unicode (e.g. \ufffd), converts the token string to bytes, and strips the prefix "incomplete unicode indicator" bytes to get the token_bytes.

Change

For this PR's "incomplete unicode" handing for "Qwen-style" BPE, the token is already bytes, so if isinstance(token_str, bytes): token_bytes = token`.