Restore pre ggplot2 v4.0 behavior of bayesplot theme setting

[jgabry]

Ensures bayesplot keeps its own default theme by stripping .Environment metadata from ggplot themes before comparison, remembering ggplot’s baseline theme, and only switching to ggplot’s theme when it truly changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.92683% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.51%. Comparing base (633addd) to head (4289605).

Files with missing lines	Patch %	Lines
R/bayesplot-ggplot-themes.R	82.92%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
- Coverage   98.62%   98.51%   -0.12%     
==========================================
  Files          35       35              
  Lines        5749     5784      +35     
==========================================
+ Hits         5670     5698      +28     
- Misses         79       86       +7     

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[jgabry]

I think there are still some kinks to work out