Feat: add Jest regression test for dhtmlx-gantt upgrades

[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
• Dependabot bumps of dhtmlx-gantt can silently break the Gantt view without showing up in the diff (v10.0.0 turned out to make window.gantt undefined at runtime).
  This adds a Jest test loading the actual built bundle (public/lib/libs.js) and asserting the gantt API surface used by gantt-helper.js is present, so the shared plugin CI catches this class of regression automatically.
  The plugin relies on GLPI core's own jest/jest-environment-jsdom install (via the CI's ../../node_modules/.bin/jest fallback), so no new devDependency was added.

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