Notebook: Add multi-objective optimization notebook and update optimizer

[cetagostini]

Description

Added a new notebook demonstrating multi-objective optimization with PyMC-Marketing. Refactored and updated budget optimizer and pytensor utilities to support multi-objective workflows. Enhanced related tests to cover new functionality and integration.

Solve a few issues which appear, now I'm testing the function.

Related Issue

• Closes #
• Related to Add model merging utilities and tests for multi-model optimization #1944

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• 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.
• If you are a pro: each commit corresponds to a relevant logical change

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📚 Documentation preview 📚: https://pymc-marketing--1973.org.readthedocs.build/en/1973/

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Codecov Report

❌ Patch coverage is 82.53968% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.46%. Comparing base (ed6ea4b) to head (b6d9b7b).

Files with missing lines	Patch %	Lines
pymc_marketing/pytensor_utils.py	76.92%	9 Missing ⚠️
pymc_marketing/mmm/budget_optimizer.py	91.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1973      +/-   ##
==========================================
- Coverage   93.54%   93.46%   -0.08%     
==========================================
  Files          67       67              
  Lines        8966     9025      +59     
==========================================
+ Hits         8387     8435      +48     
- Misses        579      590      +11     

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[cetagostini]

Notebook here: https://pymc-marketing--1973.org.readthedocs.build/en/1973/notebooks/mmm/mmm_multi_objective_optimization.html

I'll added soon into gallery.