refactor: rename register_content_models to register_publishable_models

[dwong2708]

Summary

This PR renames the function register_content_models to register_publishable_models to improve clarity and better reflect its purpose.

Context

The original name register_content_models was ambiguous and didn’t clearly convey the function’s responsibility. Since the function specifically registers models intended to be published (e.g., in learning sequences or metadata definitions), the new name register_publishable_models is more accurate and aligned with its role.

What’s Changed

• ✅ Renamed register_content_models to register_publishable_models in both implementation and usage.
• 🔄 Updated all relevant references and imports.

Testing Instructions

• Run the test suite to ensure all references to the renamed function are working correctly.
• Verify that no functional behavior has changed—this is a non-functional rename for clarity only.

Review Notes

• No logic has been altered—only a renaming refactor for clarity and maintainability.
• Please confirm naming consistency and suggest alternatives if the new name could be improved further.

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