Returns the column expressions this report type's mapping parameters accept, as ordered suggestion groups of column_name / expression pairs. Where the schema endpoints describe the columns a report produces, this endpoint describes the expressions that can go into its column mapping parameters. Each expression is complete and ready to append to the parameter's ;-separated name=expression mapping value. Suggestions are derived from the contracts the report executor resolves expressions against, so every suggestion resolves to a real value when the report runs. A report type whose mapping parameters have no suggestions yet, or which has no mapping parameters at all, returns an empty parameters list.