fix(fields): don't enforce mandatory fields on automated item creation

[MyvTsv]

• 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 !46291 !45954

Root cause

Mandatory custom fields were enforced unconditionally, even for items created without any human filling a form (tickets generated by the Mail Collector) Since there is no way to fill a mandatory custom field from an incoming email, item creation was silently rejected with "Some mandatory fields are empty".

Fix

Mandatory field enforcement in PluginFieldsContainer::validateValues() is now
skipped when any of the following applies:

• the code runs from the command line (isCommandLine())
• the code runs inside a cron task, internal or external (Session::isCron())
• the item is being created through GLPI core's "automated import" marker
  (_auto_import), set by MailCollector::buildTicket() and by recurring
  tickets, this covers the "Collect now" button too, which is a plain web
  request (neither CLI nor cron), so it wasn't covered by the first two checks
  alone.

Steps to reproduce

1. Create a "dom" container for Ticket with a mandatory custom field and no
   default value.
2. Configure a mail collector.
3. Trigger an import (cron task, or the "Collect now" button on the collector).
4. Before the fix: the ticket is rejected ("Some mandatory fields are
   empty"), even though there is no way for the sender to fill that field.
   After the fix: the ticket is created normally.