Fix: avoid crash when Safe unserialize is unavailable

[Rom1-B]

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 !45747
• The mapping and injection screens crashed with an UndefinedFunctionError because Safe\unserialize() is not shipped yet by any released GLPI 11.0.x core version. Session data is now unserialized through a shared helper that uses the Safe variant only when it actually exists, falling back to native unserialize() otherwise.

Screenshots (if appropriate):