test: pin _cut - one line for a short field, its own lines for a body

[gsneto]

Closes #73.

What changed

A new CutTests class in tests/test_lookups.py pins lookups._cut.

Why

_cut trims every look-up's text before the assistant reads it. It is pure, used everywhere, and had no test of its own.

How

Six cases, covering the behaviour the issue names:

• a short field (n < 600) collapses whitespace and newlines to single spaces;
• a body (n >= 600) keeps its own lines;
• the boundary itself: 599 collapses, 600 keeps;
• a string exactly n long is not cut;
• a longer one ends with […];
• None reads as ''.

Tests

python -m pytest tests/test_lookups.py::CutTests -q
6 passed in 3.02s

python -m pytest -q
5370 passed, 8 skipped in 337.64s

Windows 11, Python 3.12.10.

No code change - the guard is the point.