docs(bass): update notebook to the BassModel workflow

[anevolbap]

Related Issue

• Closes Update Bass notebook to use BassModel class #2590
• Closes Dual y-axes for the "Innovators vs Imitators" plots in bass notebook #1706

Description

Rewrites bass_example.ipynb around the BassModel class: model.fit with model_config/sampler_config, the plot_* methods from #2628 (plot_decomposition uses the dual y-axes asked for in #1706), out-of-sample forecasting, a save/load round-trip, and MLflow autologging from #2624. create_bass_model stays in the simulation section as the lower-level API.

Writing the notebook surfaced two bugs, fixed here with regression tests:

• BassModel.load returned plain dicts instead of Prior objects, so any loaded model crashed. Fixed with parse_model_config in __init__ plus a serialization round-trip for Scaled priors.
• Multi-product forecasting without observed data failed on a 1-D y_obs placeholder.

The notebook runs top to bottom with real sampling and passes the mocked CI runner. Stacked on #2624 and #2628; merging those first and rebasing shrinks this diff to the notebook plus the two fixes.

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format

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📚 Documentation preview 📚: https://pymc-marketing--2630.org.readthedocs.build/en/2630/

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[williambdean]

We will have to imediately port with pymc 6.0.0. Can we pause?

[anevolbap]

Sure, pausing this one. Should #2624 and #2628 wait too, or can those be reviewed on pymc 5.x? I will re-run the notebook once the 6.0.0 port lands.

[codecov]

Codecov Report

❌ Patch coverage is 99.18699% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.25%. Comparing base (649547e) to head (2bb08e5).

Files with missing lines	Patch %	Lines
pymc_marketing/bass/plotting.py	98.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2630      +/-   ##
==========================================
+ Coverage   94.19%   94.25%   +0.06%     
==========================================
  Files         100      101       +1     
  Lines       15384    15505     +121     
==========================================
+ Hits        14491    14615     +124     
+ Misses        893      890       -3     

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