Add plot_credible to plot_effects_umap

[Zethson]

Closes #1025

Cell types without a credible effect carry an effect of exactly zero (this is how credible_effects decides credibility), so they were colored as if a zero effect had been measured.
With plot_credible=True they are masked to NaN and rendered in the na_color of sc.pl.umap, leaving only credible effects colored.

Defaults to False to keep existing plots unchanged.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.72%. Comparing base (40dfeb6) to head (cdb44aa).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1063      +/-   ##
==========================================
+ Coverage   74.15%   78.72%   +4.56%     
==========================================
  Files          52       52              
  Lines        7291     7286       -5     
==========================================
+ Hits         5407     5736     +329     
+ Misses       1884     1550     -334     

Files with missing lines	Coverage Δ
src/pertpy/tools/_coda/_base_coda.py	60.66% <ø> (ø)

... and 8 files with indirect coverage changes

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