fix: match passed values case insensitively in parse_expression

[SAY-5]

• Closes parse_expression does not treat passed constants as case insensitive #800
• Executed pre-commit run --all-files or pixi run lint --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

When case_sensitive=False, parse_expression looked up **values keys with an exact-case match, so a token like Constant would fall through to a unit lookup and raise UndefinedUnitError. It now also matches the passed value names case insensitively.