Check for observed variables in the trace as well as the model

[zaxtax]

Description

This introduces the enhancement discussed in #7225

This makes it easier to use sample_posterior_predictive in model factory workflows

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)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pymc--7641.org.readthedocs.build/en/7641/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.82%. Comparing base (671d704) to head (f24a55c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7641      +/-   ##
==========================================
+ Coverage   87.41%   92.82%   +5.41%     
==========================================
  Files         107      107              
  Lines       18168    18172       +4     
==========================================
+ Hits        15881    16868     +987     
+ Misses       2287     1304     -983     

Files with missing lines	Coverage Δ
pymc/sampling/forward.py	96.21% <100.00%> (+0.06%)	⬆️

... and 10 files with indirect coverage changes