[Fix] Flattening atomic strings in groupby

[izzet]

This pull request updates a dependency and adds a new test to improve coverage for string handling in Dask aggregation utilities.

Dependency update:

• Updated the betterset dependency version requirement from >=0.1.0 to >=0.2.0 in pyproject.toml, ensuring compatibility with the latest features and bug fixes.

Testing improvements:

• Added a new test, test_unique_set_flatten_handles_strings_via_dask, to verify that the unique_set_flatten aggregation correctly handles string values when used with Dask DataFrames.

[Copilot]

Pull request overview

Updates the betterset dependency and adds a regression test to ensure unique_set_flatten correctly handles string values during multi-stage Dask DataFrame aggregations (avoiding unintended character-wise flattening).

Changes:

• Bump betterset minimum version from 0.1.0 to 0.2.0.
• Add a Dask aggregation test covering string values for unique_set_flatten.

Reviewed changes

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

File	Description
tests/utils/test_dask_agg.py	Adds coverage for unique_set_flatten with string-valued columns in a multi-stage groupby/agg pipeline.
pyproject.toml	Updates the minimum required betterset version to pick up the fix/behavior needed for correct string flattening.

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