ignore extra variables in env files

[AliSayyah]

Please describe the purpose of this pull request.
with v0.6.3, if you have extra variables in your .env file, letta fails to initiate because of the default behavior of SettingsConfDict.

How to test
Just try to import ChatMemory with extra variables in your .env file.

Have you tested this PR?
Have you tested the latest commit on the PR? If so please provide outputs from your tests.

Related issues or PRs
Please link any related GitHub issues or PRs.

Is your PR over 500 lines of code?
If so, please break up your PR into multiple smaller PRs so that we can review them quickly, or provide justification for its length.

Additional context

 |   File "/usr/local/lib/python3.12/site-packages/letta/settings.py", line 114, in <module>
gpt  |     model_settings = ModelSettings()
gpt  |                      ^^^^^^^^^^^^^^^
gpt  |   File "/usr/local/lib/python3.12/site-packages/pydantic_settings/main.py", line 167, in __init__
gpt  |     super().__init__(
gpt  |   File "/usr/local/lib/python3.12/site-packages/pydantic/main.py", line 212, in __init__
gpt  |     validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
gpt  |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gpt  | pydantic_core._pydantic_core.ValidationError: 18 validation errors for ModelSettings
gpt  | farsar_openai_api_key
gpt  |   Extra inputs are not permitted [type=extra_forbidden, input_value='sk-proj-osDZ-EC1p...BIKkifGyT40-OCcA', input_type=str]
gpt  |     For further information visit https://errors.pydantic.dev/2.9/v/extra_forbidden

[sarahwooders]

thanks for contributing this!