PhpSpreadsheet memory leak in batch XLSX export causes crash on large datasets

Problem/Motivation

When exporting large datasets to XLSX format using the batch method, DataExport::doProcessBatch() creates multiple PhpSpreadsheet\Spreadsheet objects per batch iteration but never calls disconnectWorksheets() to break their circular references.

PhpSpreadsheet objects contain circular references (Spreadsheet ↔ Worksheet ↔ Cell) that prevent PHP's refcount-based garbage collector from freeing them. Since Drupal's _batch_process() can run multiple batch operations within a single HTTP request (via its internal while-loop and timer), Spreadsheet objects from prior iterations remain in memory.

For a 1,100-row export with export_batch_size of 200 (6 iterations), each iteration loads two full Spreadsheet objects (the accumulated file and the current batch) plus creates a Writer — none of which are freed. This causes unbounded memory growth, ultimately crashing the PHP process. On PHP 8.3 with the built-in development server, this manifests as a segfault (General Protection Fault) or a silent OOM kill depending on the memory_limit setting.

The problem is in src/Plugin/views/display/DataExport.php, lines ~897–917 (the "Workaround for XLS/XLSX" block).

Steps to reproduce

1. Create a content type and populate it with 500+ nodes.
2. Create a Views Data Export display configured with:
   • Format: XLSX
   • Export method: batch
   • Batch size: 100 (or any size that requires multiple iterations)
3. Trigger the export.
4. Monitor PHP memory usage during the batch. Memory grows linearly with each batch iteration and is never reclaimed.
5. With a large enough dataset, the PHP process crashes (segfault on built-in server, or Allowed memory size exhausted on Apache/FPM).

A standalone PHPUnit test is attached that reproduces the memory growth pattern outside of Drupal's batch system.

Proposed resolution

Call disconnectWorksheets() and unset() on the $previousExcel, $currentExcel, and $objWriter variables after saving the merged result. This breaks the circular references and allows PHP's garbage collector to reclaim the memory between batch iterations.

<?php
$objWriter = IOFactory::createWriter($previousExcel, ucfirst($output_format));
$objWriter->save($vdeFileRealPath);

// Break circular references so PHP's GC can reclaim memory between
// batch iterations within a single HTTP request.
$currentExcel->disconnectWorksheets();
$previousExcel->disconnectWorksheets();
unset($currentExcel, $previousExcel, $objWriter);

This is the standard cleanup pattern recommended by the PhpSpreadsheet documentation.

Remaining tasks

Fix the code as per above suggestion...
Consider whether the same cleanup should be applied in Xls::encode() (the non-batch encoder path), which also creates Spreadsheet objects without calling disconnectWorksheets().

User interface changes

None.

API changes

None.

Data model changes

None.