Show truncated bar list labels in full on hover

What does this MR do and why?

Long labels in bar list charts are cut off at the fixed 164px label column, for example model names like accounts/gitlab/de… in the Spend tab panel "Credits by model, by capability". There was no way to read the full name.

This MR shows the full label in a tooltip when you hover a truncated label. Labels that fit show no tooltip, and the bars themselves still have no tooltip. The tooltip text is HTML-escaped.

The change is in the shared BarListChart component, so every bar list (all dashboards and GLQL barList blocks) gets it. It is frontend-only: no feature flag change, no backend change, and no docs change is needed. A spec covers the new behavior.

References

• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/631559
• Epic: https://gitlab.com/groups/gitlab-org/-/work_items/23429
• UAT comment: https://gitlab.com/groups/gitlab-org/-/work_items/23429#note_3930480839

Screenshots or screen recordings

How to set up and validate locally

1. Open /explore/analytics_dashboards/dap_impact and go to the Spend tab.
2. Hover a truncated model name in "Credits by model, by capability". The full name appears in a tooltip.
3. Hover a short model name that fits. No tooltip appears.

MR acceptance checklist

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