Select clause disappear from where clause

### Version

5.6.0

### What happened?

Jena do not handle adding a clause values in a where clause nor in an optional clause.

Here is a minimal exemple (I also give a .zip minimal maven project with .jar in it maybe it's more convenient.).

[jena-test.zip](https://github.com/user-attachments/files/24738429/jena-test.zip)

To compile and execute project just go in the decompressed directory and run
```shell
mvn clean install
java -jar target/jena-test-1.0-SNAPSHOT.jar
```
#### minimal code
```java
public static void main( String[] args )
{
Var uriVar = Converters.makeVar("uri");
Var siteAdressVar = Converters.makeVar("siteAddress");
Var cityVar = Converters.makeVar("city");

SelectBuilder select = new SelectBuilder();
select.addVar(uriVar);

SelectBuilder selectOptional = new SelectBuilder();
selectOptional.addVar(uriVar)
.addVar(cityVar);

selectOptional.addWhere(uriVar, ORG.siteAddress, siteAdressVar);
selectOptional.addWhere(siteAdressVar, VCARD4.locality.asNode(), cityVar);

System.out.println("[ optional select clause ]");
System.out.println(selectOptional);
System.out.println("----------------------------------------");

select.addOptional(selectOptional);

System.out.println("[ final select query ]");
System.out.println(select.build());
System.out.println("----------------------------------------");
}
```
---

#### output :
```
[ optional select clause ]
SELECT ?uri ?city
WHERE
{ ?uri ?siteAddress .
?siteAddress ?city}

----------------------------------------
[ final select query ]
SELECT ?uri
WHERE
{ OPTIONAL
{ ?uri ?siteAddress .
?siteAddress ?city}}

----------------------------------------
```

#### expected output :
```
[ final select query ]
SELECT ?uri
WHERE
{ OPTIONAL
SELECT ?uri ?city
WHERE
{ ?uri ?siteAddress .
?siteAddress ?city
}
}
```
as you may noticed, the `SELECT ?uri ?city` has disappeared as well as the second where clause.

#### original exemple

The original purpose was to sort on the `?city` var which is not returned. For information here is the original request i want to create (tested and validated on an RDF4J workbench server).

full request here

PREFIX org: 

PREFIX dc: 

PREFIX os-sec: 

PREFIX vcard: 

SELECT DISTINCT  ?uri ?rdfType ?rdfTypeName ?lastUpdateDate ?name ?description ?publisher ?publicationDate ?address ?locationObservationCollection

WHERE

  { ?rdfType (rdfs:subClassOf)* org:Site

    OPTIONAL

      { ?rdfType  rdfs:label  ?rdfTypeName

        FILTER langMatches(lang(?rdfTypeName), "en")

      }

    OPTIONAL

      { ?rdfType  rdfs:label  ?rdfTypeName

        FILTER langMatches(lang(?rdfTypeName), "")

      }

    GRAPH 

      { ?uri  rdf:type  ?rdfType

        OPTIONAL

          { ?uri  dc:modified  ?lastUpdateDate}

        OPTIONAL

          { ?uri  rdfs:label  ?name}

        OPTIONAL

          { ?uri  rdfs:comment  ?description}

        OPTIONAL

          { ?uri  dc:publisher  ?publisher}

        OPTIONAL

          { ?uri  dc:issued  ?publicationDate}

        OPTIONAL

          { ?uri  org:siteAddress  ?address}

      }

    OPTIONAL

      { GRAPH 

          { OPTIONAL

              { ?locationObservationCollection

                            ?uri}}}

    OPTIONAL

      { ?_organization

                  org:hasSite  ?uri}

    OPTIONAL

      { ?uri      os-sec:hasGroup       ?groups .

        ?groups   os-sec:hasUserProfile  ?userProfiles .

        ?userProfiles

                  os-sec:hasUser        }

OPTIONAL {

  {

    SELECT ?uri ?city

    WHERE {

      ?uri org:siteAddress ?address .

      ?address vcard:locality ?city .

    }

  }

}


    FILTER ( ! isBlank(?uri) )

  }

ORDER BY DESC(?city) ASC(?uri)

LIMIT   100

### Relevant output and stacktrace

```shell

```

### Are you interested in making a pull request?

None

Reproduce the issue with the attached minimal Maven project using `mvn clean install` and `java -jar target/jena-test-1.0-SNAPSHOT.jar`. Start from the `SelectBuilder` calls, especially `addOptional`, and trace how the nested builder is rendered; done means the optional block preserves its SELECT clause, projected variables, and both WHERE clauses as shown in the expected output.

Written by the indexing model from the issue text.