Fix - Massive action update on CustomAssets

[RomainLvr]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !45865
• Here is a brief description of what this PR does

Massive action "Update" was broken for Fields-plugin fields attached to Custom Assets:

1. The value input never displayed. PluginFieldsField::showSingle() looks up the field's container with a LIKE query against containers.itemtypes, but didn't escape backslashes the same way PluginFieldsContainer::getAddSearchOptions() already does. Since CustomAsset itemtypes are namespaced classes (e.g. Glpi\CustomAsset\Transmission), the query never matched, so the field never rendered, only the "Send" button showed, and submitting produced an Undefined array key warning.
2. hook.php masked the failure. plugin_fields_MassiveActionsFieldsDisplay() ignored showSingle()'s return value and always returned true, preventing GLPI core's fallback renderer from ever kicking in.
3. Multi-select "GLPI item" dropdown fields got wiped. A field of type dropdown-<Itemtype> (e.g. a multi-select of Locations) with multiple enabled was silently emptied whenever a different field in the same container was mass-updated afterwards. The plain dropdown type already had a guard against this (fixed in fix: multiple dropdown fields emptied when solution added #795 / fix(MassiveAction): fix incompatibility with multiple dropdowns #974), but it was missing on this sibling code path.

Fix

• Apply the same backslash-escaping logic in showSingle() as in getAddSearchOptions().
• Make hook.php return showSingle()'s actual result.
• Apply the same "don't blank an untouched field" guard to the dropdown-<Itemtype> multi-value branch in PluginFieldsContainer::populateData().

Test plan

• The test only passes if the patch Fix - Avoid array-to-string warnings on multi-value massive action fields glpi-project/glpi#25210 is applied otherwise, the error ‘Converting an array to a string’ causes the tests to fail.

• Added regression tests :

  • MassiveActionCustomAssetTest => field display for CustomAssets.
  • MassiveActionMultipleDropdownTest / MassiveActionRealFlowTest => plain dropdown multiple field survives a sibling field's massive update.
  • MassiveActionGlpiItemDropdownTest => dropdown-<Itemtype> multiple field survives a sibling field's massive update (fails without the fix, passes with it).

PS : i've added psalm.xml because it was missing from make test process

[stonebuzz]

see review