Add Gradient to plot posterior predictive in Plot Suite

[github-actions]

📋 Research and Implementation for #2054

This PR contains the complete workflow for issue #2054:

• ✅ Research (completed)
• ✅ Planning (completed)
• ✅ Implementation (completed)

Research

Research findings are in thoughts/shared/issues/2054/research.md

Implementation Plan

Plan is in thoughts/shared/issues/2054/plan.md

Implementation

All changes have been implemented and tested.

Ready for review and merge!

Closes: #2054

[github-actions]

✅ Implementation Complete

The implementation has been completed.

Changes:

• Implementation follows the plan in thoughts/shared/issues/2054/plan.md
• Code committed to this PR

Next steps:

• Review the implementation changes
• Run tests to verify the changes
• Merge this PR to close issue Add Gradient to plot posterior predictive in Plot Suite #2054

[codecov]

Codecov Report

❌ Patch coverage is 4.54545% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.75%. Comparing base (eb4daf4) to head (a19fe27).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc_marketing/mmm/plot.py	4.54%	21 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (eb4daf4) and HEAD (a19fe27). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (eb4daf4)	HEAD (a19fe27)
	23	21

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2074       +/-   ##
===========================================
- Coverage   92.51%   66.75%   -25.77%     
===========================================
  Files          68       68               
  Lines        9398     9420       +22     
===========================================
- Hits         8695     6288     -2407     
- Misses        703     3132     +2429     

☔ 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.