Pivot: preserve expanded rows when adding a field, sorting, or refreshing

[mahdi13]

The pivot table tracked expanded rows by TanStack's positional row ids (dot paths like 0.1.2 that index into subRows). Any config change re-partitions or re-orders the row tree, so those ids stop pointing at the same rows and the expanded state was discarded. Adding a measure or dimension, reordering row dimensions, or sorting all collapsed the tree back to the top level.

This switches expansion to value-based hierarchical keys: the dimension values from the root to a node, so a node keeps the same key across add, sort, and data refreshes.

• PivotTable sets a value-based getRowId (parent id plus this row's value), with a reserved id for the prepended grand-totals row.
• getValuesForExpandedKey and addExpandedDataToPivot resolve a key by matching each row's own value (stored under rowDimensions[0] at every depth) instead of positional indices, which also removes the totals-row offset.
• queryExpandedRowMeasureValues derives nesting depth from the key's segment count.
• getFiltersForCell, getValuesForFlatTable, getRawRowValues, the show-more row limits, and the ancestor-highlight walk all use the value keys.
• The defensive expanded = {} resets on add/sort/columns/rows are dropped. A reorder just leaves keys that match nothing (inert), and the legacy positional pivotExpanded proto field is no longer read.
• Existing unit tests updated for value-based resolution, plus a Playwright test that expands a nested row and asserts it stays open after adding a measure and sorting.

Reordering the row dimensions changes a node's value path, so those specific nodes are not restored (they were not before either). Adding a field, sorting, and refreshing now keep the tree expanded.

Closes #9781

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!