genRange crashes with ClOpen when lower bound equals upper bound

## Description

`genRange` can crash with an invalid `choose` range when the lower bound equals the upper bound and bounds are `ClOpen`.

## Location

`src/Codec/CBOR/Cuddle/CBOR/Gen.hs`, `genRange` function

## Details

When `a == b` and the range bound is `ClOpen`, the `range` helper computes `(x, pred y)`, producing an upper bound strictly less than the lower bound. This causes `choose`/`antiChoose` to crash with an invalid range.

For example, a CDDL range like `5...5` (exclusive on both ends) results in an empty range that `choose` cannot handle.

## Suggested fix

Validate that the range is non-empty after applying bounds, and emit a clear error message for empty ranges instead of crashing inside `choose`.