test: expand RDF2JSON semantic contracts

[haideriqbal]

Summary

• Add 13 packaged-CLI RDF2JSON integration tests with small local Turtle and RDF/XML fixtures.
• Assert field-level contracts for entity serialization, identity, metadata, annotations, reified and negative assertions, hierarchy, OWL relationships, and punning.
• Document the test boundary and remaining OWL/import coverage in the dataload strategy.

Verification

• Java 21: ols-shared Maven install passed.
• Java 21: dataload Maven verify passed, including all existing Java dataload tests and 13 new RDF2JSON tests.
• No production code or golden outputs changed.

Scope note

This adds direct semantic coverage across current RDF2JSON transformation stages; it does not claim exhaustive combinations of anonymous class expressions, import topologies, or multi-parent cycles. The existing full-pipeline golden tests remain in place.