fix(typing): improve accuracy of type propagation for collections

[moe-ad]

Related to #2695. It is a short change, but I want to explain the "why" clearly for future reference.

Recent studying of python type system and playing around the codebase made me realize a couple of things that are still wrong with the existing type propagation:

• A custom collection like StringFieldsCollection is a actually recognized by type checkers as a collection of class objects rather than class instances. For example, this is why string_field.get_entity_data() below is expecting two arguments (the instance and the index) instead of one (the index):
  
• Type propagation does not even work at all on other IDEs like vscode (I tested fix: design and type hint of custom collections #2695 on PyCharm alone 😞)
  
  
  

In summary, turns out:

• The second problem is caused by not correctly identifying the subtype parameter of collection_factory method as what actually drives the type parameterization of that method. This is solved by using a type parameter specific to the method. See here.
• The first problem is solved annotating subtype with Type[Type Param] rather than just Type Param. Doing that ensures Collection[Type Param] in the return annotation is correctly interpreted as a collection of instances and not of class objects.

With this change, things work accurately now across both PyCharm and vscode.

[PProfizi]

That's awesome @moe-ad! Thanks for that!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.46%. Comparing base (00d243b) to head (e551960).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2724      +/-   ##
==========================================
+ Coverage   84.38%   84.46%   +0.08%     
==========================================
  Files          92       92              
  Lines       10899    10900       +1     
==========================================
+ Hits         9197     9207      +10     
+ Misses       1702     1693       -9     

[github-actions]

Some tests with 'continue-on-error: true' have failed:

• PyDPF-Post API tests on ubuntu-latest

Created by continue-on-error-comment