Add tests for variable substitution and transforms #32

Description:
Test variable replacement and transformation logic thoroughly.

Tasks:

• Test {{variable}} substitution
• Test nested variables
• Test stripCurrency transform
• Test stripNonNumeric transform
• Test regexReplace transform
• Test parseAsNumber

Acceptance Criteria:

• All transform types tested
• Edge cases (empty, null, undefined) covered
• Regex patterns tested
• Number parsing edge cases covered