Automatically handle the group_by_column field on the ArrowChart

[palewire]

No description provided.

[Copilot]

Pull Request Overview

This PR refactors the ArrowChart class to automatically derive the group-by-column feature flag from the presence of a groups_column value, rather than requiring it to be set explicitly. The group_by_column boolean field is removed in favor of inferring this setting from whether groups_column is None or not.

• Replaced explicit group_by_column boolean field with automatic derivation from groups_column presence
• Added new groups_column field to store the column name for grouping arrows
• Updated serialization to set group-by-column flag based on groups_column being non-None

Reviewed Changes

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

File	Description
datawrapper/charts/arrow.py	Removed group_by_column field, added groups_column field, and updated serialization/deserialization logic to automatically handle the group-by-column feature flag
tests/integration/test_arrow_chart.py	Updated tests to use groups_column parameter and verify its value instead of the removed group_by_column boolean

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