Add unittest suite and GitHub Actions CI workflow

[Copilot]

No test infrastructure existed for the LEKCut library. This adds a full unittest suite covering every supported model and a CI workflow to run it automatically.

Tests (tests/test_lekcut.py)

25 tests across all seven tokenizer models (deepcut, attacut-sc, attacut-c, oskut, sefr-ws1000, sefr-tnhc, sefr-best), each covering:

• Basic tokenization — output is a non-empty list whose concatenation equals the input
• Known output assertions (deepcut)
• Empty string → []
• NotImplementedError raised for unknown model names

from lekcut import word_tokenize

result = word_tokenize("ทดสอบการตัดคำ", model="deepcut")
assert result == ["ทดสอบ", "การ", "ตัด", "คำ"]

result = word_tokenize("", model="deepcut")
assert result == []

CI (.github/workflows/tests.yml)

Runs python -m unittest discover -s tests -v on push/PR to main across Python 3.9–3.12, with permissions: contents: read.

Bug fixes

deepcut.py and attacut.py lacked the empty-string guard already present in oskut.py, causing an ONNX runtime crash on empty input. Added if not text: return [] to their tokenize methods.