expand test coverage

[malcolmbarrett]

No description provided.

[Copilot]

Pull request overview

This PR expands the package’s test coverage by adding new unit tests for core cause/component helpers and by introducing visual snapshot tests for causal_pie() / causal_pie_necessary() plots.

Changes:

• Add additional testthat coverage for causify(), components(), necessary_causes(), and sufficient_causes().
• Introduce vdiffr-based visual snapshot tests (with generated SVG snapshots) for plot outputs and themes.
• Update DESCRIPTION to include vdiffr in Suggests.

Reviewed changes

Copilot reviewed 5 out of 17 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/testthat/test-components.R	Adds test cases for necessary/sufficient cause helpers and components() behavior with/without U.
tests/testthat/test-causify.R	Adds tests for fraction splitting logic, labeling format, and sequential U naming.
tests/testthat/test-causalpie.R	Adds structural ggplot tests plus vdiffr visual snapshot coverage for pies, necessary pies, and themes.
tests/testthat/helper-vdiffr.R	Adds a wrapper for vdiffr::expect_doppelganger() with skip logic.
tests/testthat/_snaps/causalpie/*.svg	Adds vdiffr snapshot artifacts for plot rendering expectations.
DESCRIPTION	Adds vdiffr to Suggests.

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