Add test for boundary values in level-0/03-temperature-converter

## Description
The temperature converter should handle boundary values like absolute zero (-273.15 C), boiling point (100 C), and freezing point (0 C). Add test cases for these.

## Acceptance Criteria
- [ ] Add tests for 0 C, 100 C, and -273.15 C conversions
- [ ] Verify correct Fahrenheit output for each
- [ ] Existing tests still pass