A single field parsed from a --fields spec, e.g. title:String or
age:int? (the trailing ? marks the field nullable), or inferred from a
JSON sample (where isModel flags fields whose type is a generated model
that exposes fromJson / toJson).
Parses a --fields value like "title:String, done:bool, age:int?" into a
list of FieldSpec. Whitespace is tolerated; a missing type defaults to
String. Returns an empty list for null/blank input.
