Ensure support for all o models, including o3 and o4-mini

[deepdive101]

At this point, it is just wrong for OpenAI to release models without updating tiktoken at the same time.

[DavidAmat]

when it is expected to add o4- in MODEL_PREFIX_TO_ENCODING in tiktoken/model.py ? they are basically using the same tokenizer as o3, so no reason for not updating tiktoken right away

[kaimatzu]

Yeah, it's all o200k_base at this point. But they should specify this explicitly on model release lol. It's confusing for developers.

[hauntsaninja]

This is fixed in recent versions of tiktoken