Add state counts for planning views work items

What does this MR do and why?

Add state counts for planning views work items

Skip fetching closed/opened counts when workItemPlanningView flag is enabled. Adds excludeStateCounts parameter to GraphQL queries and displays total count only in planning view UI.

• Add @skip directive for closed/opened fields in state counts queries
• Add slot to issuable list for custom count rendering
• Update query handler mocks to conditionally return fields
• Add test coverage for planning view state count behavior

Changelog: added EE: true

References

Relates to #560177 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable work_item_planning_view flag
2. Visit project or group work items
3. See the counts displayed as per the filter

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