validate_date_range
ChatGPTValidate that start_date and end_date are present, parseable as ISO 8601 calendar dates in YYYY-MM-DD format, and that start_date is not later than end_date. Returns fixed structured output with deterministic results and the error contract codes missing_field, invalid_value, out_of_scope, and internal_error. Boundaries: Only validates ISO 8601 calendar dates in YYYY-MM-DD format. Does not infer time zones, times, locales, or natural-language dates. Does not repair ambiguous inputs or call external services. Does not validate business calendars, holidays, or booking rules.