Returns the field definitions for an expense report that an approver is reviewing. Use this endpoint instead of the regular expense-report fields-initialize endpoint when rendering the approver's review screen — it loads the report by {expenseReportId} and assembles the response in approval context. Requires the caller to be assigned as an approver in the organization and to be authorised on the specific expense report; otherwise the endpoint returns 404. The 404 is intentionally identical to "expense report not found" so approval-access can't be probed.