feat: Add MistralProvider

[mattzh72]

Description

Add support for MistralProvider. We will create a follow up PR for actually querying the Mistral models.

Testing

Added unit testing to test listing models.

(letta-py3.12) mattzhou@Matts-MacBook-Pro MemGPT % poetry run pytest -s -vv tests/test_providers.py::test_mistral
=================================================================================== test session starts ===================================================================================
platform darwin -- Python 3.12.6, pytest-8.3.3, pluggy-1.5.0 -- /Users/mattzhou/Library/Caches/pypoetry/virtualenvs/letta-Mdwnr1Ge-py3.12/bin/python
cachedir: .pytest_cache
rootdir: /Users/mattzhou/MemGPT/tests
configfile: pytest.ini
plugins: asyncio-0.23.8, order-1.3.0, anyio-3.7.1
asyncio: mode=Mode.AUTO
collecting ... Initializing database...
collected 1 item                                                                                                                                                                          

tests/test_providers.py::test_mistral ['open-mistral-7b', 'mistral-tiny', 'mistral-tiny-2312', 'open-mistral-nemo', 'open-mistral-nemo-2407', 'mistral-tiny-2407', 'mistral-tiny-latest', 'open-mixtral-8x7b', 'mistral-small', 'mistral-small-2312', 'open-mixtral-8x22b', 'open-mixtral-8x22b-2404', 'mistral-small-2402', 'mistral-small-2409', 'mistral-small-latest', 'mistral-medium-2312', 'mistral-medium', 'mistral-medium-latest', 'mistral-large-2402', 'mistral-large-2407', 'mistral-large-latest', 'codestral-2405', 'codestral-latest', 'codestral-2405-blue', 'codestral-2405', 'codestral-latest', 'codestral-mamba-2407', 'open-codestral-mamba', 'codestral-mamba-latest', 'pixtral-12b-2409', 'pixtral-12b', 'pixtral-12b-latest', 'mistral-embed']
PASSED