Fixing issue with multiplicity=1 for simple type associations in Reader/Writer generators

• Understand the issue: when an association has multiplicity=1 and type is a simple type like String, generators incorrectly call parseString/writeString/updateString methods that don't exist
• Create test case to reproduce the issue
• Fix Xpp3ReaderGenerator to handle simple type associations with multiplicity=1
• Fix Xpp3WriterGenerator to handle simple type associations with multiplicity=1
• Fix SnakeYamlReaderGenerator to handle simple type associations with multiplicity=1
• Fix SnakeYamlWriterGenerator to handle simple type associations with multiplicity=1
• Fix StaxReaderGenerator to handle simple type associations with multiplicity=1
• Fix StaxWriterGenerator to handle simple type associations with multiplicity=1
• Fix JDOMWriterGenerator to handle simple type associations with multiplicity=1
• Verify all tests pass
• Rebased on master and resolved conflicts with Root -> rootElement rename

Summary

The issue was that when an association field has multiplicity="1" and the type is a simple type (like String), the code generators incorrectly tried to call methods like parseString(), writeString(), and updateString() which don't exist.

Root Cause

The generators assumed all associations with multiplicity="1" were references to model classes and generated calls to parseClassName() or writeClassName() methods. They didn't account for simple types like String which are allowed in associations but aren't model classes.

Solution

Added a check in all affected generators to determine if the association type is a class in the model using isClassInModel(). If it's not a model class, the code now uses the primitive field handling logic instead.

Rebase on Master

• Successfully rebased on latest master (commit bca1253)
• Resolved conflict in JDOMWriterGenerator.java where root variable was renamed to rootElement
• All tests pass after rebase

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