fix(fuzz): reduce mem budget to prevent timeouts

[thevilledev]

Fixes OSS-Fuzz timeout #893 caused by expressions like "-817810..178" creating ~818K element arrays that exceeded the 60s timeout when passed to fmt.Sprintf for string formatting.

I had this as an alternative, basically to modify the fuzz test helper to skip formatting large input:

func Func() expr.Option {
    return expr.Function("fn", func(params ...any) (any, error) {
        // Avoid expensive string formatting of large slices
        if len(params) == 1 {
            if arr, ok := params[0].([]int); ok && len(arr) > 10000 {
                return fmt.Sprintf("fn([%d elements])", len(arr)), nil
            }
        }
        return fmt.Sprintf("fn(%v)", params), nil
    })
}

But the mem budget modification felt like a cleaner option.