fix(serving): resolve rust tokenizer from ProcessorMixin in streaming generation (#45368)
ProcessorMixin subclasses (e.g. Qwen3VLProcessor) expose the fast tokenizer
at .tokenizer, not ._tokenizer. Use getattr() to handle both ProcessorMixin
and PreTrainedTokenizerFast when extracting the rust tokenizer backend for
DirectStreamer and CBStreamer.
Fixes #45362
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>