fix: pivot row dimension width reset

[djbarnwal]

Fixes pivot table row dimension column widths resetting unexpectedly after table state changes.

https://linear.app/rilldata/issue/APP-802/pivot-table-columns-widths-reset-unexpectedly-when-switching-tabs-or

Changes

• Persist nested pivot row dimension widths in the same way measure widths are persisted.
• Key the persisted row dimension width by the first row dimension, so width survives:
  • switching tabs
  • expanding or collapsing nested pivot rows
  • adding or removing unrelated pivot columns
• Add unit coverage for stable nested row dimension width key behavior.

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!

[AdityaHegde]

Doesnt this need a reset between explores?

[djbarnwal]

Yes, have added a scope key for different explore and canvas instances. Given a canvas can have multiple pivot tables, the scope key contains the component id as well in those cases.