fix(destinationfield): prevent stale value leak across form answers

[DaviidMM]

Checklist before requesting a review

• I have performed a self-review of my code.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.

Description

• Fixes Form Helpdesk: custom field value overwritten by stale $value from previous QuestionTypeItemDropdown answer #1201

• In PluginFieldsDestinationField::applyConfiguratedValueToInputUsingAnswers(),
  $value is assigned only inside the QuestionTypeItemDropdown branch and is
  never reset between iterations of the foreach ($answers as $answer) loop.
  On a later iteration targeting a non-dropdown field (number, text,
  textarea, richtext, url, date, datetime), the expression
  $value ?? $answer->getRawAnswer() returned the stale $value from the
  previous iteration, silently overwriting the user's typed value with the
  items_id of the earlier item-dropdown answer.

• $value has no meaning outside the QuestionTypeItemDropdown branch (the
  glpi_item / dropdown branches read items_id directly from the current
  answer), so the fall-through else must always read from the current answer:

  - $input[$field_name] = $value ?? $answer->getRawAnswer();
  + $input[$field_name] = $answer->getRawAnswer();

• Single-line change in inc/destinationfield.class.php. No behavior change
  for QuestionTypeItemDropdown, glpi_item or dropdown target field types.

• Reproduction and root cause analysis are detailed in Form Helpdesk: custom field value overwritten by stale $value from previous QuestionTypeItemDropdown answer #1201.