Fix SpecificAssetId JSON for SelfManagedEntity

[sankalpsthakur]

Description of Changes

SelfManagedEntity specificAssetIds loaded from XML were empty objects, so GET /submodels/{id}/submodel-elements/{path} returned [{}] instead of name, value, semanticId, and externalSubjectId.

AAS4J's Entity XML mixin does not declare the specificAssetId item wrapper that AssetInformation already has. BaSyx now applies that mixin when loading XML/AASX, and registers a JSON mixin so those fields serialize on the HTTP ObjectMapper.

Related Issue

Fixes #933

BaSyx Configuration for Testing

No special configuration. Covered by unit tests:

• TestEntitySpecificAssetIdJsonSerialization — HTTP ObjectMapper, SelfManagedEntity with one SpecificAssetId does not emit {}
• TestEntitySpecificAssetIdsXmlLoad — XML environment load keeps name/value/semanticId and JSON-serializes them

AAS Files Used for Testing

self_managed_entity_specific_asset_ids.xml — minimal SelfManagedEntity with one SpecificAssetId (name, value, semanticId), matching the issue repro.

Additional Information

I have signed the Eclipse Contributor Agreement. The commit includes Signed-off-by: Sankalp Thakur <sankalphimself@gmail.com>.

AI/LLM disclosure

• AI coding tools (including Grok and/or Codex agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and ran the reported local tests before submitting.
• This submission is original work of authorship under the project CLA / contributor terms; AI output was not pasted unreviewed.

[aaronzi]

Thanks for trying to fix this problem. I still have to reject this PR since it's only a workaround for the actual problem. The fix needs to be done in AAS4J. More details can be found in the issue that is referenced in this PR.