how to get the last id inserted in JSF and Primefaces?

It's very simple. Order descending by code and limit the results to one. In this way, you will always get the last code inserted.

String sql = "SELECT p.codigo FROM Producto p ORDER BY p.codigo DESC";
TypedQuery<String> query = em.createQuery(sql, String.class);
query.setMaxResults(1);
// cuidado, lanza NoResultException si no hay resultados
String lastCode = query.getSingleResult();

If in the debug you can observe the new assigned value. Take a look at the update attribute of the action or button that opens the dialog, panelgrid or whatever, give it an id and refresh it from the action (update attribute) so that update=": idDelElemento"

Regardless of your logic to get the next Id, do not directly update the dialog and include a container such as outputpanel :

<p:dialog header="Registro de ......" widgetVar="wdlgDatos" id="dlgDatos">

      <h:form id="formDialog">

        <p:outputPanel id="output">

           <p:panelGrid columns="2">

               <p:outputLabel  value="codigo" />
               <p:outputLabel  value="#{pruebaBean.idprueba}"/>

               <p:outputLabel  value="des" />
               <p:inputText  value="#{pruebaBean.prueba.nombre}"/>


               <p:commandButton value="#{pruebaBean.accion}" actionListener="#{pruebaBean.operar()}" oncomplete="PF('wdlgDatos').hide();" update=":frm:data"/>
               <p:commandButton value="Cancelar" immediate="true" oncomplete="PF('wdlgDatos').hide();"/>

            </p:panelGrid>           

        </p:outputPanel>

    </h:form>

And update it in the commandButton with something like the following:

update= ": formDialog: output"

Greetings!

Put an id to your form that is inside the dialog: id="idFormRegistro"

<p:dialog header="Registro de ......" widgetVar="wdlgDatos" id="dlgDatos">

           <h:form id="idFormRegistro">
         <p:panelGrid columns="2">

         <p:outputLabel  value="codigo" />
         <p:outputLabel  value="#{pruebaBean.idprueba}"/>

           <p:outputLabel  value="des" />
           <p:inputText  value="#{pruebaBean.prueba.nombre}"/>


        <p:commandButton value="#{pruebaBean.accion}" actionListener="#{pruebaBean.operar()}" oncomplete="PF('wdlgDatos').hide();" update=":frm:data"/>
        <p:commandButton value="Cancelar" immediate="true" oncomplete="PF('wdlgDatos').hide();"/>

        </p:panelGrid>           

        </h:form>

and on your new button would be as follows:

<p:commandButton value="Nuevo" oncomplete="PF('wdlgDatos').show()" actionListener="#{pruebaBean.setAccion('Registrar')}"  onclick="#{pruebaBean.numeracionPrueba()}" update=":idFormRegistro"/>

The problem is in your XHTML, you must update the dialog but in the form put an id to the form of the dialog so <h:form id="formDialog"> and in the button that sends to call the dialog add in the update the id of the form formDialog <p:commandButton value="Nuevo" oncomplete="PF('wdlgDatos').show();" actionListener="#{pruebaBean.setAccion('Registrar')}" onclick="#{pruebaBean.numeracionPrueba()}" update=":formDialog"/>

 <f:event type="preRenderView" listener="#{pruebaBean.listar('F')}" />     
      <p:growl id="msj" autoUpdate="true" />
            <h:form id="frm">

                <p:commandButton value="Nuevo" oncomplete="PF('wdlgDatos').show();" actionListener="#{pruebaBean.setAccion('Registrar')}"  onclick="#{pruebaBean.numeracionPrueba()}" update=":formDialog"/>
                <p:dataTable id="data" value="#{pruebaBean.lstPrueba}" var="prueba">
                    <p:column headerText="Codigo">
                        <p:outputLabel value="#{prueba.idprueba}"/>
                   </p:column>  

            <p:column headerText="descripcion">
                <p:outputLabel value="#{prueba.nombre}"/>
            </p:column>   


                </p:dataTable>
            </h:form>
      <p:dialog header="Registro de ......" widgetVar="wdlgDatos" id="dlgDatos">

           <h:form id="formDialog">
         <p:panelGrid columns="2">

         <p:outputLabel  value="codigo" />
         <p:outputLabel  value="#{pruebaBean.idprueba}"/>

           <p:outputLabel  value="des" />
           <p:inputText  value="#{pruebaBean.prueba.nombre}"/>


        <p:commandButton value="#{pruebaBean.accion}" actionListener="#{pruebaBean.operar()}" oncomplete="PF('wdlgDatos').hide();" update=":frm:data"/>
        <p:commandButton value="Cancelar" immediate="true" oncomplete="PF('wdlgDatos').hide();"/>

        </p:panelGrid>           

        </h:form>**texto en negrita**

You are concatenating to a String (test) another String (1) in the following code

this.idprueba=this.prueba.getIdprueba()+1;

Given in test id are stored text values such as:

• P001
• P002
• P003
• ...

You should extract the whole part of that code and treat it as a number to be able to increase by one unit.