Updated import-related classes to enhance JSON serialization/deserialization, removed unused enums, added new validation codes, and streamlined legacy format conversion in `ImportResultConverter`. These changes improve code maintainability and align with new API requirements.

Introduced a boolean parameter to determine whether counters should be updated in the parseLineResults method, ensuring flexibility for error handling scenarios. Removed redundant @NotNull annotations to simplify code. These changes improve clarity and functionality of the import utility.

This update introduces logic to track the last inode during imports and makes it accessible as an optional value. Adjustments include storing, retrieving, and converting the last inode to maintain consistency across processes. This enhances import result tracking and improves data handling.

Adjusted validation parameters to align with updated requirements and fixed assertion to correctly reflect the expected error count. This ensures tests accurately validate warnings and errors, improving code reliability.

Replaced JSONObject-based map transformation with ObjectMapper for better consistency and reduced dependency. Simplified `transformToMap` method implementation and updated the documentation accordingly. Additionally, minor code formatting improvements were made in ImportContentletsProcessor.

Replaced manual iteration and filtering with stream-based processing to retrieve unique fields. This improves readability and maintains the same functionality with cleaner code.

Replaced repetitive calls to 'result.data()' with the 'data' variable to improve code readability and maintainability. This change reduces redundancy and ensures consistent access to the processed data throughout the affected methods.

This PR represents the final stage of the Import Content response
improvements, where the `ImportContentletsProcessor` now leverages the
new structured response format while maintaining full backward
compatibility with existing Struts implementations.

### Changes Overview

* Updated `ImportContentletsProcessor` to use new response format 
* Added converter to maintain backward compatibility with Struts actions
* Improved error handling and validation message organization
* Enhanced documentation and code organization

### What's Not Changed

We intentionally kept several aspects unchanged to ensure stability:
* Core import logic remains untouched
* Legacy response format is still fully supported
* Existing workflow processing behavior
* Current transaction handling
* Permission checking mechanisms

While working on this code, we identified several potential improvements
that were intentionally left for future iterations to keep the scope
focused on the immediate needs.

This PR is related to: #30882
This PR fixes: #31339