2D: Add QPieModelMapper tests for in-place updates

Cover the behavior of the optimized QPieModelMapper:

- series edits with a model that resets from insertRows() and
  removeRows() keep the slices and don't emit removed
- external resets update the slices, and add or remove slices when the
  row count changes
- unchanged resets and unchanged setters emit nothing and keep the
  slices
- custom slice color and exploded state survive changes to first,
  count and the sections, and a model reset
- shifting the values or labels section in the other orientation
  updates the slices in place
- setSeries(nullptr), setModel(nullptr) and changing the series detach
  the previous one from the model

Pick-to: 6.12
Task-number: QTBUG-151153
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Change-Id: I45920a9060de5c096db3e47222f39cb227e4bb4a
Reviewed-by: Tomi Korpipaa <tomi.korpipaa@qt.io>