Fix nested fields not json parsed when they are complex #3971

[rastaclaus]

Change Summary

composite fields of nested objects are now json-parsed

Related issue number

fix pydantic/pydantic-settings#41

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[rastaclaus]

All tests ok, please review, thanks (=

[hramezani]

Thanks @rastaclaus for the patch 👍

I think we already have another fix in #4216 that seems more complete than yours.
Also, it seems your proposed solutions will fail by below test:

def test_nested_env_complex_values(env):
    class SubSubModel(BaseSettings):
        dvals: Dict

    class SubModel(BaseSettings):
        vals: List[str]
        sub_sub_model: SubSubModel

    class Cfg(BaseSettings):
        sub_model: SubModel

        class Config:
            env_prefix = 'cfg_'
            env_nested_delimiter = '__'

    env.set('cfg_sub_model__vals', '["one", "two"]')
    env.set('cfg_sub_model__sub_sub_model__dvals', '{"three": 4}')

    assert Cfg().dict() == {'sub_model': {'vals': ['one', 'two'], 'sub_sub_model': {'dvals': {'three': 4}}}}

    env.set('cfg_sub_model__vals', 'invalid')
    with pytest.raises(SettingsError, match='error parsing JSON for "cfg_sub_model__vals'):
        Cfg()