Support OpenAI OSS Models with Tiktoken tokenizer

[tarekgh]

Fixes #7491

[Copilot]

Pull Request Overview

This PR adds support for OpenAI OSS (Open Source Software) models using the Tiktoken tokenizer by introducing a new encoding type o200k_harmony and supporting the gpt-oss model family.

• Introduces a new O200kHarmony encoding with specialized special tokens for OSS models
• Adds comprehensive test coverage for the new tokenizer functionality
• Expands model name mappings to include various OpenAI model variations and fine-tuned models

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs	Adds O200kHarmony encoding support, new special tokens, and expanded model mappings for gpt-oss and fine-tuned models
test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs	Adds comprehensive test coverage for the new OSS tokenizer including encoding tests and special token validation

[tarekgh]

CC @stephentoub @ericstj

[stephentoub]

Thanks

[stephentoub]

We should also add gpt-5:
openai/tiktoken@5818d56#diff-0d973848bd229418209db2c46c86167000845592ca6b98fad215c21c317bc494R12