Exclude index columns for pivot for count/sum agg

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📝 Summary

This was caught in CI tests.

Fix pivot fill_null(0) casting boolean index columns

handle_pivot applied nw.all().fill_null(0) to every column — including index columns. On boolean index columns this either silently casts the dtype to Int32 or raises an InvalidOperationError depending on the Polars version. Use nw.exclude(*index_columns).fill_null(0) so only the aggregated value columns are filled.

Adds a regression test in test_handlers.py.

🔍 Description of Changes

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• Tests have been added for the changes made.
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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Mar 19, 2026 8:43am

[Copilot]

Pull request overview

This PR fixes a pivot transform edge case where fill_null(0) was being applied to pivot index columns, which could cast boolean index columns to integers (or error) in some Polars versions. The handler now fills nulls only for the aggregated value columns, and adds a regression test to prevent dtype regressions.

Changes:

• Update handle_pivot to apply fill_null(0) only to non-index (aggregated) columns for count/sum.
• Add a Polars regression test ensuring boolean index column dtypes are preserved for count pivots.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
marimo/_plugins/ui/_impl/dataframes/transforms/handlers.py	Excludes pivot index columns from fill_null(0) to avoid dtype casting/errors.
tests/_plugins/ui/_impl/dataframes/test_handlers.py	Adds a regression test asserting boolean index dtype is preserved after a count pivot.

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