MCR-3650 Refactor MCRConfigurableInstanceHelper into multiple classes

[toKrause]

Link to jira.

Relies on #2861.

Pull Request Checklist (Author)

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Ticket & Documentation

• The issue in the ticket is clearly described and the solution is documented.
• Design decisions (if any) are explained.
• The ticket references the correct source and target branches.
• The fixed-version is correctly set in the ticket and matches the PR's target branch (main).

Feature & Improvement Specific Checks

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• Does this change affect existing applications, data, or configurations?
  • Yes: Is a migration required? If yes, describe it.
  • Breaking change is marked in the commit message.

Testing

• I have tested the changes locally.
• The feature behaves as described in the ticket.
• Were existing tests modified?
  • Yes: explain the changes for reviewers.

MCR Conventions & Metadata

• [x MCR naming conventions are followed
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  • Old API is deprecated or a migration is documented.
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• Java license headers are added where necessary.
• Javadoc is written for non-self-explanatory classes/methods (Clean Code).
• All configuration options are documented in Javadoc and mycore.properties.
• No default properties are hardcoded — all set via mycore.properties.

Multi-Repo Considerations

• Is an equivalent PR in MIR required?
  • If yes, is it already created?