Add test coverage for `clean()`

[sbfnk]

clean() currently has no dedicated tests, which allowed #249 to go undetected.

The function handles several distinct code paths that should be tested:

1. Country name normalisation - 2-letter ISO, 3-letter ISO, and full country names
2. Non-numeric age entries - entries without numbers should become NA
3. "Under X" format - e.g., "Under 1" → "0-1"
4. Age unit conversion - months/weeks to years
5. Age range parsing - "20-30" → part_age_est_min = 20, part_age_est_max = 30
6. Exact age column creation - part_age_exact from numeric ages

A regression test for (5) should be added to #250, but more comprehensive coverage would help prevent future regressions.