main: fix configuration model validation after CLI args merge

[airwoodix]

The bug reported in #1435 seems to originate in the Config._validate_base_class method: in Pydantic v2, @model_validator(mode="after") decorates an instance method instead of a class method that receives the raw values (the case for mode="before"). Therefore, except when called to alter set_args in Config.merge_args, the values argument is of type Config and "base_class" not in values is always true.

This MR is an attempt at fixing the issue, for Pydantic v2 only for the moment.

TODOS:

• also fix the issue with Pydantic 1.x
• add regression test for Wrong parent class for pydantic V2 root models  #1435.

Closes #1435.

[koxudaxi]

@model_validator(mode="after") decorates an instance method instead of a class method that receives the raw values (the case for mode="before").

@airwoodix
Thank you for creating this PR.
I didn't know the changes.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e9b6edf) 100.00% compared to head (71690da) 100.00%.
Report is 690 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #1448     +/-   ##
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  Coverage   100.00%   100.00%             
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  Files           11        32     +21     
  Lines         1020      3563   +2543     
  Branches       201       838    +637     
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+ Hits          1020      3563   +2543     

Flag	Coverage Δ
unittests	99.60% <99.54%> (?)

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Files Changed	Coverage Δ
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/dataclass.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/imports.py	100.00% <100.00%> (ø)
... and 22 more

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