Add MergeReportsWidget to group the report widgets

Phil Hughesrequested to merge
ph/mergeReportsWidgetGroup into master

What does this MR do and why?

Renders each report widget as a level 2 row inside one collapsible widget that totals the counts the rows report. Behind the merge_request_reports_widget_group feature flag.

References

Screenshots or screen recordings

Screenshot_2026-09-15_at_12.14.09

Screenshot_2026-09-15_at_12.20.35

How to set up and validate locally

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