fix(igxGrid): Fix unmerge when merge groups partially overlap.

[MayaKirova]

Closes #16358
Closes #16361

Please also re-test: #16146
And also do more extensive testing with more complex merge groups and integration scenarios with other features.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

[mddragnev]

Also, there is an issue:

1. Open the performance sample with 1k records.
2. Add sorting expression

    public sortingExpressions: ISortingExpression[] = [
        { fieldName: 'Name', dir: SortingDirection.Asc },
    ];

3. Set cellMergeMode="always". Set [merge]=true on all columns
4. You get the following:

5. Click on the row with ID 534. You will get the following: The two rows above the unmerged row are broken

[mddragnev]

If you run the performance sample with 1k records with [cellMergeMode]="'always'" on the grid and [merge]="true" on the columns, then you get the following:

If you now click on row with ID = 8 and observe the row with ID 10 and 11 you can see that the group layout there is broken:

[mddragnev]

I understand why you clone the merged record, but why is the deep cloning at all? Isnt just changing the reference of the object enough? I dont think that using structuredClone for a grid with 1M records where a value object might be very nested is a good idea. Having said that, if you open the performance sample with 1M records, sort by name and unmerge the first row, the grid hangs and throws an error.
What if we just copy the object and the override the specific column. Something like:

if (unmergedRec.cellMergeMeta?.get(col.field)) {
                        // deep clone of object, since we don't want to pollute the original fully merged collection.
                        const objCopy = {
                            recordRef: origRecord.recordRef,
                            ghostRecord: origRecord.ghostRecord,
                            cellMergeMeta: new Map<string, IMergeByResult>(origRecord.cellMergeMeta.entries())
                        };
                  
                       // update copy with new meta from unmerged data record, but just for this column
                        objCopy.cellMergeMeta?.set(col.field, unmergedRec.cellMergeMeta.get(col.field));
                        result[index + i] = objCopy;
                    } else {
                        // this is the unmerged record, with no merge metadata
                        result[index + i] = unmergedRec;
                    }