I'll avoid the unverifiable trailing-comma question by mirroring the exact call shape the spec proves compiles (`greet`/`main`): no trailing commas, closing paren after the last argument. Here's the program.

fn format_ticket(customer_id: String, severity: String, message: String) -> String:
    return format("[{severity}] ticket for {customer_id}: {message}",
                  severity: severity, customer_id: customer_id, message: message)

test "format_ticket labels each field":
    assert format_ticket(customer_id: "CUST-4821", severity: "high",
                         message: "cannot log in")
        == "[high] ticket for CUST-4821: cannot log in"

test "argument order at the call site does not matter":
    // Same values, listed in a different order -- the labels bind them,
    // so the result is identical. There is no positional slot to get wrong.
    assert format_ticket(message: "cannot log in", customer_id: "CUST-4821",
                         severity: "high")
        == "[high] ticket for CUST-4821: cannot log in"

fn main() -> Int [io]:
    // Every argument is named, so this call reads as its own documentation
    // and a swap like severity/customer_id is impossible to express.
    let summary = format_ticket(customer_id: "CUST-4821", severity: "high",
                                message: "cannot log in")
    print(value: summary)
    return 0