FIX / Mandatory fields blocking ticket creation from a GLPI form

[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 GLPI form cannot create a ticket when a mandatory additional field is not part of the form: the creation fails with "Some mandatory fields are empty". The field is expected to stay out of the form while remaining mandatory on the ticket itself.

This is a follow-up to #1275 (merged), which only clarified the error message and did not remove the blocking.

Items created by a form destination are now exempted from the mandatory check, like the other automated creations handled in #1274. The exemption also covers the update done right after the creation to link the item to the form. On the ticket form, a mandatory dropdown that was never filled is now rendered with its empty option, so saving shows the mandatory error instead of doing nothing.

Screenshots:

[Mary-Clb]

Some points following my commit :

• The exemption now only covers mandatory fields the form does not provide; a mapped field left empty blocks the creation again.
• dropdown-* fields use dropdown_value as suggested.
• The input token is replaced by a static state in PluginFieldsDestinationField, holding the fields provided by the form.
• That state is cleared once the destination is done, so later updates of the ticket are validated as usual.
• Tests added for the mapped field left empty, the update after the destination, and the empty option on mandatory dropdowns.