PivotGrid: restore Field Panel operations after dataSource reassignment

[nightskylark]

PR Title
PivotGrid: restore Field Panel operations after dataSource reassignment

PR Description
This PR fixes a regression where Field Panel interactions stop working after dataSource is reassigned (for example: initialize with dataSource: null, then set a new PivotGridDataSource).

Root cause

• On dataSource change, PivotGrid called _fieldChooserBase._dispose().
• The disposed dxPivotGridFieldChooserBase instance stayed attached to the element.
• On next render, _createComponent(...) reused that stale instance, so Field Panel logic (groupFilter, sorting, header filter) still referenced the old data source.

What was changed

• Added _disposeFieldChooserBase() in m_widget.ts to fully clean up FieldChooserBase:

1. call _dispose()
2. remove the instance from element data
3. remove the component name from dxComponents
4. reset _fieldChooserBase reference

• Replaced direct _fieldChooserBase._dispose() calls with _disposeFieldChooserBase().

Tests

• Added regression test: Field panel operations should work after dataSource reassignment.
• The test verifies:

1. Field Panel group is valid for drag-and-drop (groupFilter === true)
2. sorting from Field Panel updates current data source state
3. header filter opens with items