Address minor test gaps for DAP dashboard

What does this MR do and why?

Minor test gaps found while auditing the DAP Impact analytics dashboard area, plus one change that came out of it.

• GLQL panels now read the visualization's own data.query.dateRange and use it when the dashboard has no date range picker. If both are set, the dashboard picker wins — which is what the other analytics data sources already do.
• This makes the option usable rather than fixing a current bug. The GLQL data source never read the setting, so a panel-level date range was ignored with no error, and no dashboard relies on it today.
• The explore visualization schema's GlqlQuery definition now permits dateRange — it was the only query type missing it, which is why the option was unreachable.
• 5 new tests: 2 fail without the change, 3 pin the precedence so it can't be silently inverted later.
• The GLQL presenter routing spec covered ten display types but not divergingBarChart. Routing and the "is this supported?" check are independent, so a broken routing condition for a supported type would render nothing with no error.

References

Screenshots or screen recordings

How to set up and validate locally

VUE_VERSION=3 yarn jest spec/frontend/analytics/analytics_dashboards/components/data_sources/glql_spec.js spec/frontend/glql/components/presenters/data_spec.js
bundle exec rspec ee/spec/validators/json_schemas/analytics_dashboard_schemas_spec.rb ee/spec/services/analytics/custom_dashboards/system_dashboards_loader_spec.rb

Check the divergingBarChart guard by breaking its condition in data.vue and confirming the new spec fails.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.