Normalise and validate a list of state codes: trims, upper-cases, de-duplicates
(preserving order) and rejects anything not among the 16 known codes with a
HochwasserzentralenValidationError — so a typo never becomes a silently-dropped
filter that returns the full nationwide set.
Normalise and validate a list of state codes: trims, upper-cases, de-duplicates (preserving order) and rejects anything not among the 16 known codes with a HochwasserzentralenValidationError — so a typo never becomes a silently-dropped filter that returns the full nationwide set.