Add GPT-5.1 support with o200k_base encoding

[magkue]

Add GPT-5.1 Model Support

Fixes Add GPT-5.1 with the o200k_base encoding in tiktoken/model.py #464

Changes

• Added GPT-5.1 model mapping: Maps gpt-5.1 to o200k_base encoding in MODEL_TO_ENCODING and MODEL_PREFIX_TO_ENCODING

Impact

Enables tiktoken to properly tokenize text for GPT-5.1 model using o200k_base encoding as requested in #464

[eshaanpathak]

Thank you! @hauntsaninja, can you please review this PR?

[Itsindigo]

Hey @magkue -- could we update this for other models in the 5.x paths, 5.1, 5.2, 5.3, 5.4 etc -- you might need some more generalised pattern matching logic