Fix: fill mandatory ticket content in escalation test

[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 #N/A
• testTaskGroupEscalation created its ticket with empty content, which GLPI core rejects as a missing mandatory field once the test reaches the "solved ticket" step.
• This left an unhandled session error message and made the two data sets covering solve/reject-solution escalation fail.
• The ticket now gets non-empty content so those data sets pass again; no plugin behavior changed.

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