fastokens backend patch.
When VLLM_USE_FASTOKENS=1 is set, fastokens.patch_transformers() swaps the inner Rust tokenizer of every HF fast tokenizer loaded afterwards with the fastokens shim and rebinds tokenizers.decoders.DecodeStream so the streaming detokenizer accepts the shim. The patch is process-global and idempotent, so it applies to any tokenizer mode that ends up loading an HF fast tokenizer (hf, deepseek_v32, deepseek_v4, qwen_vl, …).