feat(pinia-orm): STI Models can only be retrieved through parent

[CodeDredd]

🔗 Linked issue

closes #1735

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

It's more an enhancment than a bugfix. The problem in this issue was that the child models didn't had a default @type value
on which a where clause was filtering. Added now entity as fallback if no default value for type is defined.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.71%. Comparing base (807fa36) to head (5a2da94).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1835   +/-   ##
=======================================
  Coverage   99.71%   99.71%           
=======================================
  Files          88       88           
  Lines        6027     6030    +3     
  Branches      511      512    +1     
=======================================
+ Hits         6010     6013    +3     
  Misses         13       13           
  Partials        4        4           

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