Fix/reference cycle guard signed

[stiebitzhofer]

Summary

• prevent circular referredSemanticId chains in DefaultReference by rejecting self/indirect cycles during assignment
• add unit tests that cover self and indirect cycles plus happy-path acyclic chains
• include JUnit dependency in the model module to compile the new tests
• unblocks SAMM CLI AAS generation (esmf-sdk#802) by preventing the StackOverflow from circular references

Background

Jackson XML serialization blew up with a StackOverflow when Reference.referredSemanticId contained a cycle (e.g., Reference -> Reference -> same Reference). The model allowed such graphs because the setter did not guard against cycles. This PR keeps the model acyclic so downstream serializers do not need custom circular-reference handling. This removes the root cause behind SAMM CLI’s infinite recursion during AAS generation (see esmf-sdk#802).

Implementation Notes

• cycle detection uses identity (not equals) to avoid false positives from structurally equal references
• the guard walks the referredSemanticId chain once and fails fast with a clear IllegalArgumentException message
• tests cover self-reference, indirect cycles, and a valid chain

Testing

• mvn -pl model test -Dmaven.repo.local=./.m2