perf(core): avoid unnecessary re-serialisation

[mttrbrts]

Serialization to JSON is costly (especially for large models). This change removes a redundant serialization step in the validateMetaModel function. The tests don't rely on this behaviour and the signature is unchanged. All validation happens in the deserialization step.

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname