validatePrefix function
Validate a prefix used for listing/deleting.
Same rules as validateKey but allows trailing / (required for
prefix operations) and allows empty string (list everything).
Implementation
void validatePrefix(String prefix) {
if (prefix.isEmpty) return;
// Strip trailing slash for validation, then validate as key
final normalized = prefix.endsWith('/')
? prefix.substring(0, prefix.length - 1)
: prefix;
if (normalized.isEmpty) return;
validateKey(normalized);
}