Fix issues with model graph

[ricardoV94]

Closes #7397

Also when working on #7820 I noticed our detection of observed variables was based on old behavior where only Cast operations were allowed in the graph. This PR fixes that as well

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📚 Documentation preview 📚: https://pymc--7844.org.readthedocs.build/en/7844/

[ricardoV94]

@williambdean do you want to review?

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.91%. Comparing base (965534a) to head (e9dcc27).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc/model/core.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7844   +/-   ##
=======================================
  Coverage   92.91%   92.91%           
=======================================
  Files         107      107           
  Lines       18286    18301   +15     
=======================================
+ Hits        16991    17005   +14     
- Misses       1295     1296    +1     

Files with missing lines	Coverage Δ
pymc/model_graph.py	84.43% <100.00%> (-0.65%)	⬇️
pymc/model/core.py	93.35% <50.00%> (-0.13%)	⬇️

... and 1 file with indirect coverage changes

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[jessegrabowski]

lgtm

[williambdean]

Thank you, you OSS legend