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.

Screenshots (if appropriate):

[stonebuzz]

Suggested change

	"jest": "^30.0.5",
	"jest-environment-jsdom": "^30.0.5",
	"script-loader": "^0.7.2",

jest and jest-environment-jsdom are used by the test script but absent from devDependencies.

so jest --colors will fail with "command not found" in any environment that does not already have GLPI core's ../../node_modules/.bin/jest.

[Rom1-B]

npm run walks up parent node_modules/.bin dirs automatically, so npm test in plugins/gantt already resolves jest from GLPI core's ../../node_modules/.bin/jest ; same fallback pattern the Makefile already uses for phpunit/phpstan.

CI doesn't even go through npm test: continuous-integration.yml calls the binary directly with an explicit fallback (node_modules/.bin/jest → ../../node_modules/.bin/jest).

Only breaks if you run the plugin fully standalone outside a GLPI checkout ; same limitation already accepted for the PHP tools. So nothing to change.