Migrate ImportContentletsProcessor to Use New Import Result Format While Maintaining Legacy Support

[jgambarios]

Parent Issue

#30882

Task

Implement structured import results in the ImportContentletsProcessor while maintaining backward compatibility with the legacy HashMap format for the existing admin UI.

Current Situation

• The ImportUtil has been refactored to support a new structured format for import results
• The legacy format (HashMap<String, List>) is still required by the admin UI
• ImportContentletsProcessor currently uses the legacy format directly

Objective

Enhance ImportContentletsProcessor to:

1. Use the new structured format internally for better type safety and maintainability
2. Convert results to legacy format only when needed for backward compatibility
3. Maintain existing functionality without breaking changes

Proposed Objective

Core Features

Proposed Priority

Priority 2 - Important

Acceptance Criteria

• ImportContentletsProcessor uses structured format
• Legacy format support maintained for admin UI
• No changes required in existing UI code
• All existing functionality preserved
• Unit tests pass and maintain coverage
• No regression in error handling or reporting

Related issues

#31236
#31237