Add tests for theme/aesthetic helper functions

[utkarshpawade]

Fixes #523

• Added return type checks (expect_s3_class, incomplete theme validation) for plot_bg(), panel_bg(), facet_bg(), legend_move(), legend_text()
• Added on = FALSE → element_blank verification for all three background helpers
• Added composition tests with real bayesplot plots (ppc_hist, ppc_dens_overlay, ppc_dens)
• Added overlay_function return type check (LayerInstance, not theme) and plot composition
• Added legend_move("none") equivalence with legend_none()
• Added test for stacking multiple helpers on a single plot

[Copilot]

Pull request overview

Adds expanded test coverage for bayesplot theme/aesthetic convenience helpers, with checks for return types and compatibility when added to real bayesplot ggplot objects.

Changes:

• Added tests asserting helper outputs are incomplete themes and map on = FALSE to element_blank().
• Added composition tests ensuring helpers and overlay_function() can be added to real bayesplot plots.
• Added a NEWS entry documenting the added tests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
tests/testthat/test-convenience-functions.R	Adds new testthat cases around theme helper return types and plot composition.
NEWS.md	Documents newly added tests for theme/aesthetic helpers.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.15%. Comparing base (44c3e5c) to head (3e6dfd0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #524   +/-   ##
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  Coverage   99.15%   99.15%           
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  Files          35       35           
  Lines        5920     5920           
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  Hits         5870     5870           
  Misses         50       50           

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[jgabry]

Thanks @utkarshpawade. I think the legend_move("none") test is worth keeping, since that behavior is explicitly documented and it guards a real contract of the helper.

For the rest, I’m not convinced the added tests are giving us much benefit. Most of these helpers are very thin wrappers around ggplot2::theme() or ggplot2::stat_function(), and the existing tests already cover the main behavior by checking the returned objects directly. The new composition tests mostly confirm that ggplot can still add a valid theme/layer object to a plot, which is more ggplot behavior than bayesplot behavior. The extra on = FALSE checks also seem redundant with the existing expectations that already compare against element_blank() themes.

So my preference would be to keep the legend_move("none") test and drop the others, to avoid adding test volume without much additional protection.