Function parse_select 

pub fn parse_select(raw: &str) -> Result<Vec<String>, CanonicalError>

Parse $select string into a list of field names. Format: “field1, field2, field3, …” Field names are case-insensitive and whitespace is trimmed.

Errors

Returns a CanonicalError if the select string is invalid. Axum renders it via IntoResponse for CanonicalError; the canonical_error_middleware fills instance / trace_id on the way out.