respect env_prefix when extra allowed

[zzstoatzz]

closes #237 and adds regression test

example of updated behavior

setup

» tree -a
.
├── .env
└── settings.py

.env

FOO_X=something
FOO_Y=something_else

settings.py

from pydantic_settings import BaseSettings, SettingsConfigDict


class FooSettings(BaseSettings):
    model_config = SettingsConfigDict(
        extra='allow',
        env_file='.env',
        env_prefix='foo_'
    )
    
    x: str

settings = FooSettings()

print(f'{getattr(settings, "y", None)=}')
print(f'{getattr(settings, "foo_y", None)=}')

usage

» python settings.py
getattr(settings, "y", None)='something_else'
getattr(settings, "foo_y", None)=None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (965d1b4) 97.63% compared to head (abcc165) 97.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
+ Coverage   97.63%   97.65%   +0.02%     
==========================================
  Files           5        5              
  Lines         422      426       +4     
  Branches       89       90       +1     
==========================================
+ Hits          412      416       +4     
  Misses          7        7              
  Partials        3        3              

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[hramezani]

Thanks @zzstoatzz for reporting and preparing the fix