Add weight field to iteration report label-filter query

What does this MR do and why?

On the iteration report page, the Weight column goes blank for every issue once you group/filter by label — the column header stays but no values populate.

Root cause: the page uses two different GraphQL operations depending on whether a label filter is active, and only one of them requests weight:

• Default view → IterationIssues query → iteration_issues.fragment.graphql → includes weight
• Grouped-by-label view → IterationIssuesWithLabelFilter query → iteration_issues_with_labels.fragment.graphql → did not include weight

iteration_report_issues.vue defines the Weight column statically regardless of which query ran, and maps weight: issue?.weight || null. Since the label-filtered query never requested weight, that field was always undefined for every row once a label filter was applied.

This MR adds weight to iteration_issues_with_labels.fragment.graphql, in the same position it already appears in iteration_issues.fragment.graphql.

     assignees {
       nodes {
         ...User
       }
     }
+    weight
     workItemType {
       id
       iconName
     }

Relates to #536394 (closed) (open since April 2025, missed 9 milestones — a customer independently hit this and reported it to Support; reproduced on 18.10 and 19.2).

Screenshots

Before	After

Still needed before this is ready to merge

• Frontend test coverage confirming weight renders correctly in the grouped/label-filtered view (see ee/spec/frontend/iterations/components/iteration_report_issues_spec.js, which already has findWeightsForIssue helpers for the default view)
• Confirm no GraphQL query complexity budget issue — iteration_issues_with_label_filter.query.graphql is already flagged for high complexity in #584292
• Screenshot/recording of the fix
• Changelog trailer review

Opening as Draft so the fix approach can be reviewed before investing in full test coverage.

MR acceptance checklist

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

• I have included changelog trailers, or none are needed
• I have added tests, or none are needed
• I have added documentation, or it's not needed