Add Japanese tokenizer (fugashi) and minimal unit test

[supernaiter]

What does this PR do?

This PR adds minimal support for Japanese prompt tokenization.

What's included:

• A Japanese tokenizer utility (tokenize_jp) using fugashi + unidic-lite
• A Unicode-based language detector (is_japanese_text) to support lang="auto"
• Minimal unit tests for tokenizer correctness
• setup.py updated with extras_require["ja"] to optionally install Japanese dependencies

This is the first step toward enabling Japanese prompt compression, designed to be self-contained and safe to merge.
Future work (e.g., integration into compress_prompt) will follow as separate PRs.

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Fixes: N/A

Before submitting

• [x] This PR is a new feature.
• [x] Changes are backward-compatible.
• [x] Tests for new functionality are included.
• [x] No documentation changes are needed at this stage.

Who can review?

@iofu728 @SiyunZhao — this is a minimal PR for Japanese support. Would love your input before we follow up with lang="ja" integration.

[supernaiter]

@microsoft-github-policy-service agree

[supernaiter]

Hi @iofu728 @SiyunZhao — This is a minimal PR to support Japanese prompt tokenization. All tests passed, CLA is signed, and the PR is self-contained.

Would love your feedback or approval when convenient. Thanks!