Pin the system dashboard tiling in the loader spec

What does this MR do and why?

Follow-up from !254441 (comment 3820285012). Adds a spec to system_dashboards_loader_spec.rb that walks every view (or the top-level panels) of every shipped system dashboard and asserts no two panels occupy the same grid cell, so a future row trim cannot quietly leave two panels overlapping. The failure message names the dashboard, panel, row and column.

I checked the assertion against both shipped dashboards (dap_impact, duo_and_sdlc_trends) before adding it; both tile cleanly today.

References

• Resolves #628694
• Origin: !254441 (merged)

Differences

Before: Two panels with overlapping gridAttributes in a system dashboard YAML pass CI.

After: The loader spec fails and points at the overlapping panel.

How to set up and validate locally

1. Run bundle exec rspec ee/spec/services/analytics/custom_dashboards/system_dashboards_loader_spec.rb.
2. Temporarily change a panel's yPos in ee/lib/gitlab/analytics/dashboards/system/dap_impact.yaml to collide with its neighbour; the new example fails.

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