FIX / Clarify mandatory field error when a GLPI form cannot provide it

[Mary-Clb]

• 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 !46413
• A mandatory field in a "dom" container blocks the creation of any ticket that does not submit it, including tickets created from a native GLPI form that has no question mapped to that field. The admin only saw a generic "Some mandatory fields are empty" message, with no hint that the form itself was the cause.

The mandatory check is kept, but when nothing was submitted for the container, a second message now states that the form or source creating the item does not provide these fields, and that they must be mapped or made optional.

Warning the admin when the form is saved would have been better, but the GLPI form framework has no hook for a plugin to validate a destination configuration, so it would require changing GLPI core.

Screenshots :

[Rom1-B]

$not_submitted is actually $item->isNewItem() at the only call site (line 2055), not a generic "nothing submitted" flag. Worth a name/doc that reflects the real coupling, so a future caller doesn't pass true outside the "new item + dom container + empty" case this relies on?