When using the builtin "general" AssemblerGroup, do we need to create a custom FileManger assembler?

### Version

5.3.0

### Question

I'm currently trying to create a group of nested models with the Assembler, and am using a document manager to manage multiple ontologies for an OntModel. I'm running into the issue where the assembler group is looking for an assembler for the DocumentManager ja:fileManager object, but that doesn't exist on the general assembler group.

Looking through the code, I want to think it would use the DocumentManagerAssembler's "getFileManager" method to assemble the FileManager, but it's looking for a root assembler on the group itself.

Assembly.ttl
```
@prefix ja: .
@prefix lm: .
@prefix ja: .
@prefix rdfs: .

a ja:UnionModel ;
ja:subModel .

a ja:UnionModel ;
ja:subModel .

a ja:OntModel ;
ja:ontModelSpec .

a ja:DocumentManager ;
ja:fileManager [ ja:locationMapper ] .

lm:mapping [ lm:altName "file:/path/to/v1.vocab.ttl" ] ;
lm:name "file:v1.vocab.ttl" .

a ja:OntModelSpec ;
ja:documentManager .

a ja:OntModel ;
ja:ontModelSpec .

a ja:DocumentManager ;
ja:fileManager [ ja:locationMapper ] .

lm:mapping [ lm:altName "file:/path/to/v2.vocab.ttl" ] ;
lm:name "file:v2.vocab.ttl" .

a ja:OntModelSpec ;
ja:documentManager .

a ja:UnionModel ;
ja:subModel ,
.
```

[DocumentManagerAssembler#getFileManager](https://github.com/apache/jena/blob/jena-5.3.0/jena-core/src/main/java/org/apache/jena/assembler/assemblers/DocumentManagerAssembler.java#L45)

Start with DocumentManagerAssembler.java, especially getFileManager, and trace how the builtin general AssemblerGroup resolves the inline ja:fileManager in Assembly.ttl. Reproduce the nested OntModel and DocumentManager setup from the issue, then determine whether the general group can assemble it without a custom FileManager assembler. Done means the expected nested models and document managers assemble successfully, or the required limitation is clearly documented.

Written by the indexing model from the issue text.