fix: nested array stored in DB for readonly dropdown

[TarekRemo]

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.

Description

• It fixes ticket !44062.

• Steps to reproduce:

  • Create a container with at least two multiple-dropdown custom fields : one set as readonly and one editable.
  • Open an item that has a value saved for the readonly dropdown (e.g. ["3"]).
  • Change the value of the editable dropdown and save the form.
  • Reload the page : a PHP error is thrown.

• Root cause:

  • The dropdownField Twig macro pre-appends [] to the field name for all multiple inputs (e.g. plugin_fields_donefielddropdowns_id[]).
  • For readonly fields, Dropdown::show() renders hidden inputs and appends another [], producing plugin_fields_donefielddropdowns_id[][].
  • When the form is saved, PHP parses this double-bracketed name as a nested array ([['3']] instead of ['3']), which gets JSON-encoded as [["3"]] and stored corrupted in the database.
  • On the next page load, Dropdown::show() receives the decoded nested array, iterates over it, and tries to use an array as an array key inside isset(), triggering a PHP 8 TypeError.

• Fix:

  • Before json_encode-ing a multiple field value for storage, flattenScalars() collapses any nested arrays into a flat list of scalars, preventing the corruption from ever reaching the database.
  • The same flattening is applied at display time so that values already corrupted in the database before this fix are handled.