Fixes #452 - Adding support for populate_by_name

[hozn]

Adds support for populate_by_name model config and a handful of tests to validate this functionality.

• I wasn't 100% sure on whether the preference was to modify existing tests (which, for example, ensure that populate-by-name is not happening), but I opted to create new tests for this code path specifically.
• I only added tests for env and dotenv. Let me know if more coverage is needed on some of the alternative options.

[hramezani]

Thanks @hozn

[mcgfeller]

I wonder if the following is the intended behavior, as it differs from pydantic.BaseSettings in Pydantic 1.x?

If the value is both present in the configuration and the aliased environment variable, it fails with Extra inputs are not permitted. I don't want to allow extra inputs, because that basically omits validations of mis-spelled settings.

pydantic_settings 2.7.0, pydantic 2.10.3

""" Demonstrate problem of value both present in the settings and in the aliased environment variable """
import os
import pydantic
import pydantic_settings

class Settings(pydantic_settings.BaseSettings):

    model_config = pydantic.ConfigDict(populate_by_name=True)
    my_setting: str = pydantic.Field(default='TEST', validation_alias='ENV_VARIABLE')

""" Test that the settings are correctly loaded """

s = Settings()
assert s.my_setting == 'TEST'

s = Settings(my_setting='PROD')
assert s.my_setting == 'PROD'

os.environ['ENV_VARIABLE'] = 'FAIL'
try:
    s = Settings() # uses env variable
    assert s.my_setting == 'FAIL'

    s = Settings(my_setting='PROD')  # Fails in pydantic_settings 2.7.0, pydantic 2.10.3
    assert s.my_setting == 'FAIL'  # Passes using pydantic.BaseSettings in Pydantic 1.x
finally:
    os.environ.pop('ENV_VARIABLE', None)