ElementInfoManager overwrites a form element's own #value_callback declaration

Problem/Motivation

ElementInfoManager::buildInfo() calls each form element plugin's getInfo(), then unconditionally overwrites #value_callback for any FormElementInterface implementation with [$definition['class'], 'valueCallback']. Any element that declares its own #value_callback in getInfo() has that value silently discarded.

This masked a separate bug in \Drupal\Core\Render\Element\File::getInfo(). It declared #value_callback as [[static::class, 'valueCallback']], an incorrectly double-wrapped array. Because the overwrite above always ran first, this invalid value was never actually used, so the bug had no visible effect until now.

This surfaced while wiring \Drupal\file\Element\ManagedFile::getInfo() to declare its own #value_callback (see #3616654: [PP-2] Convert ManagedFile::valueCallback() logic into an injectable service), since that declaration was likewise being discarded.

Steps to reproduce

Proposed resolution

Change ElementInfoManager::buildInfo() to only assign the default #value_callback when the element's own getInfo() has not already declared one. Remove the redundant, malformed #value_callback declaration from File::getInfo(), since File::valueCallback() already exists and is exactly what the default assignment would wire up.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet