Add CausalPy experiment integration

[cetagostini]

Description

Introduce a CausalPy bridge for MMM lift-test calibration:

1. add pymc_marketing.mmm.experiment with ExperimentType, ExperimentResult and run_experiment (lazy-imports causalpy and converts results to lift-test DataFrame).
2. Export ExperimentResult/ExperimentType from mmm package and integrate into MMM by adding MMM.experiment and MMM.add_lift_test convenience methods to run experiments and add their results as lift-test measurements.
3. Add optional dependency "experiment = ["causalpy>=0.7.0"]" to pyproject.toml
4. Comprehensive unit tests under tests/mmm/test_experiment.py.

Related Issue

• Closes #
• Related to Make causalpy a property in the MMM multidimensional class #2178

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--2283.org.readthedocs.build/en/2283/

[codecov]

Codecov Report

❌ Patch coverage is 93.67089% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.05%. Comparing base (c717344) to head (f412258).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc_marketing/mmm/multidimensional.py	50.00%	3 Missing ⚠️
pymc_marketing/mmm/experiment.py	97.22%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2283   +/-   ##
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  Coverage   93.04%   93.05%           
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  Files          78       79    +1     
  Lines       12228    12307   +79     
=======================================
+ Hits        11378    11452   +74     
- Misses        850      855    +5     

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