Enable RDF/XML Round-Trip Tests

[remiceres]

Description

Implement parser-serializer round-trip tests for RDF/XML format once parser becomes available.

Current Status

• Serializer: ✅ Available
• Parser: ❌ Missing (blocked by PR feature/86-RDFXML-parser #176)
• Tests: Ready but disabled

Dependency

Blocked by RDF/XML parser implementation: PR #176

Tasks

Once parser is available:

1. Remove @Disabled annotations from RDFXMLCircularTest.java
2. Run round-trip tests and fix any compatibility issues
3. Verify XML namespace/prefix handling works correctly