Make do interventions shared variables by default

[ricardoV94]

This behavior should be more intuitive, since do interventions create Data containers, and users expect those to be mutable by default. This also alleviates some issues related to sample_posterior_predictive that currently does not look for sources of mutability on constants:

Closes #6977 (I tested the MWE locally)
Related to #7069

We could consider something similar for pm.observe but that doesn't create data containers at all right now, so it's a tad less relevant and would require more changes.

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📚 Documentation preview 📚: https://pymc--7596.org.readthedocs.build/en/7596/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.83%. Comparing base (7c369c8) to head (d14cf22).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7596      +/-   ##
==========================================
- Coverage   92.84%   92.83%   -0.01%     
==========================================
  Files         106      106              
  Lines       17719    17723       +4     
==========================================
+ Hits        16451    16454       +3     
- Misses       1268     1269       +1     

Files with missing lines	Coverage Δ
pymc/model/transform/conditioning.py	95.91% <100.00%> (+0.17%)	⬆️

... and 1 file with indirect coverage changes