Returns the field definitions needed to render a form for creating or editing an expense report — the container that groups expense records before submission. Pass expenseReportId (query) to seed the response with an existing report's values for editing; omit it to start from a blank report for creation. The set of fields surfaced depends on the organization's configuration — for example, the recipient field is only included when the organization has at least one expense-management-service recipient with open-email enabled.