feat: DH-21183: Add Context Menu on Pivot ColumnBy sources

[vbabich]

No description provided.

[Copilot]

Pull request overview

Adds a dedicated context menu for Pivot “columnBy source” headers (negative column indexes), enabling users to sort/filter/copy from those source headers while suppressing menus on non-source grouped headers.

Changes:

• Introduces PivotContextMenuHandler to build and trigger context-menu actions (sort, filter, copy) for pivot column source headers.
• Adds comprehensive Jest coverage for context menu behavior and action generation.
• Registers the new handler via usePivotMouseHandlers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
plugins/pivot/src/js/src/PivotContextMenuHandler.ts	New grid mouse handler that triggers a context menu on pivot column source headers and wires sort/filter/copy actions.
plugins/pivot/src/js/src/PivotContextMenuHandler.test.ts	New unit tests covering context-menu triggering/suppression and action behavior for sort/filter/copy.
plugins/pivot/src/js/src/hooks/usePivotMouseHandlers.ts	Adds the new context-menu handler to the pivot grid mouse handlers list.

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