test: add initial test infrastructure with 21 unit tests for utility functions

[ayazhankadessova]

Summary

Sugar-AI currently has no test infrastructure. This PR adds the first test suite covering pure utility functions that do not require model loading or GPU access.

What's Added

• tests/__init__.py — makes tests a Python package
• tests/test_utils.py — 21 tests across 4 function groups:

Function	Tests	What's covered
extract_answer_from_output	10	None/empty inputs, Answer/Child-friendly markers, precedence, fallback
format_docs	3	Single doc, multiple docs joined, empty list
combine_messages	2	String passthrough, non-string conversion
RAGAgent._extract_after_prompt	6	Prompt removal, EOS token trimming, double-newline truncation with length threshold

Why These Functions

These are pure utility functions with no side effects — they don't need a model loaded, GPU access, or network calls. This makes the tests fast (~4 seconds) and runnable on any contributor's machine.

Running

pip install pytest
python -m pytest tests/ -v

All 21 tests pass:

tests/test_utils.py::TestExtractAnswerFromOutput::test_none_input_returns_empty PASSED
tests/test_utils.py::TestExtractAnswerFromOutput::test_empty_list_returns_empty PASSED
...
======================== 21 passed, 2 warnings in 3.81s ========================

AI Disclosure

This PR was developed with AI assistance (Claude Code). All tests were manually verified against the actual function behavior before submission.