Expand scientific notation floats in number_to_words

[gaoflow]

Summary

• expand float inputs rendered by Python in scientific notation before parsing digits
• keep string inputs on the existing string parsing path
• add regression coverage for small float values such as 0.000001

Fixes #226

Verification

• python -m pytest tests/test_pwd.py -k numwords -q
• python -m pytest -q
• python -m compileall inflect tests
• python -m ruff format --check inflect/init.py tests/test_pwd.py
• git diff --check upstream/main...HEAD

Note: python -m ruff check inflect/init.py tests/test_pwd.py still reports pre-existing UP006/UP007/UP035 typing modernization diagnostics across inflect/init.py that are unrelated to this change.