Replace c(mult, add) with expansion() in scale expand args

[utkarshpawade]

Fixes #484

Summary
•Replaced all old-style expand = c(mult, add) numeric vector arguments with the expansion() helper across helpers-gg.R, mcmc-•diagnostics.R, mcmc-diagnostics-nuts.R, mcmc-distributions.R, and mcmc-parcoord.R
•The expansion() helper was introduced in ggplot2 3.3.0 and is the documented modern approach; the old c(mult, add) format is still supported but discouraged
•Brings these files in line with the style already used in mcmc-intervals.R and ppc-loo.R.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (b98d1e3) to head (bf8996e).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #485      +/-   ##
==========================================
- Coverage   98.70%   98.66%   -0.04%     
==========================================
  Files          35       35              
  Lines        5857     5857              
==========================================
- Hits         5781     5779       -2     
- Misses         76       78       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jgabry]

The cleanup looks only partial to me. The direct scale_*() calls were updated to expansion(), but there are still calls to helper functions using the old numeric c(mult, add) form. Since dont_expand_y_axis() just forwards expand into scale_y_continuous(), the old-style expand is still being used indirectly.

[jgabry]

Changing the default here is good, but there are still places in the code where we call e.g.

dont_expand_y_axis(c(0.005, 0))

in which case this is still using the old behavior.

[jgabry]

Thanks! I added a few regression tests to check the panel expansion. If everything passes we can merge.