fix: split CJK from Latin regex tokens

[cat0825]

Description

The default regex tokenizer merged CJK characters with adjacent Latin words into a single token, producing token boundaries that did not align with the source character intervals. This change splits CJK runs from Latin runs so each is emitted as its own word token while preserving exact source-character offsets.

• Prevent the default regex tokenizer from merging CJK text with adjacent Latin words.
• Keep CJK spans classified as word tokens while preserving exact source character intervals.
• Add regression coverage for Latin/CJK and accented-Latin/CJK boundaries.

Fixes #334

Bug fix

How Has This Been Tested?

$ python -m pytest tests/tokenizer_test.py -q
$ python -m pytest tests/chunking_test.py tests/resolver_test.py -q
$ python -m pyink langextract/core/tokenizer.py tests/tokenizer_test.py
$ git diff --check

Also ran python -m pytest -q. Collection is blocked in this local environment because google.genai is not importable for the Gemini provider tests (tests/annotation_test.py, tests/gemini_retry_test.py, tests/inference_test.py, tests/provider_schema_test.py); those are unrelated to this change.

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page, and I either signed the Google Individual CLA or am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes.
• I have followed Google's Python Style Guide and ran pylint over the affected code.

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