FormSet Bugs

[JohananOppongAmoateng]

This PR addresses multiple long-standing edge cases in django-polymorphic related to formsets and admin customization. Specifically, it improves how polymorphic formsets handle child form configuration, polymorphic_ctype initialization, and fieldset extensibility in subclassed models.

The changes aim to make polymorphic forms more predictable, composable, and aligned with standard Django behavior, while removing the need for fragile workarounds in downstream projects.
comprehensive documentation about the ManyToMany limitation with polymorphic inline forms to address GitHub issue #294.
Closes #578
closes #549
Closes #472
Closes #363
Closes #294

[codecov]

Codecov Report

❌ Patch coverage is 90.38462% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.28%. Comparing base (06729c3) to head (c649b5f).

Files with missing lines	Patch %	Lines
src/polymorphic/formsets/models.py	80.95%	1 Missing and 3 partials ⚠️
src/polymorphic/admin/childadmin.py	96.77%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #756      +/-   ##
==========================================
+ Coverage   83.98%   84.28%   +0.29%     
==========================================
  Files          27       27              
  Lines        1611     1654      +43     
  Branches      253      269      +16     
==========================================
+ Hits         1353     1394      +41     
  Misses        196      196              
- Partials       62       64       +2     

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