Expand logging test cases for sample_prior_predictive and add return type overloads

[nataziel]

expand logging test cases for sample_prior_predictive & add return type overloads

Description

Expanded logging test cases to clarify expected behaviour/purpose of logged message in sample_prior_predictive

Added return type overloads while I'm in there for useability (and to remove a bunch of instances of assert isinstance(trace, az.InferenceData) from my own code 😂)

Related Issue

• Closes BUG: pm.sample_prior_predictive reports it will sample all volatile basic RVs even when var_names is supplied #7703

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):

---

📚 Documentation preview 📚: https://pymc--7707.org.readthedocs.build/en/7707/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.67%. Comparing base (d1aff0b) to head (36e861e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7707   +/-   ##
=======================================
  Coverage   92.67%   92.67%           
=======================================
  Files         107      107           
  Lines       18319    18323    +4     
=======================================
+ Hits        16977    16981    +4     
  Misses       1342     1342           

Files with missing lines	Coverage Δ
pymc/sampling/forward.py	96.32% <100.00%> (+0.06%)	⬆️

[nataziel]

Happy to split the overload addition to a separate PR if it's preferrable

[ricardoV94]

Thanks @nataziel