fix: replace array_merge for upload

[ishetnogferre]

Problem

When importing asset arrays where some assets already exist and others need to be uploaded, the use of array_merge() operations destroys the relationship between array positions and their corresponding sub-field data. This causes alt text, custom fields, and other asset metadata to be applied to the wrong assets.

Root Cause

The issue occurs in Assets.php:parseField() where:

1. Line ~225: $foundElements = array_merge($foundElements, $ids) destroys array position mapping
2. Lines ~250, ~256: Upload results are merged without preserving original positions
3. Sub-field population: populateFields() maps field data using array keys that no longer correspond to original positions

Replace array_merge() operations with position-preserving array assignments:

Key Changes

1. Initialize with position structure: $foundElements = array_fill_keys(array_keys($value), null)
2. Preserve positions during asset finding: $foundElements[$key] = $id instead of array_merge()
3. Maintain positions during uploads: $foundElements[$uploadKey] = $uploadedElements[0]
4. Keep array keys during cleanup: Remove only null values, preserve indices

Related issues

Fixes #1722