langchain-mistralai - make base URL possible to set via env variable for ChatMistralAI (#25956)

Thank you for contributing to LangChain!


**Description:** 

Similar to other packages (`langchain_openai`, `langchain_anthropic`) it
would be beneficial if that `ChatMistralAI` model could fetch the API
base URL from the environment.

This PR allows this via the following order:
- provided value
- then whatever `MISTRAL_API_URL` is set to
- then whatever `MISTRAL_BASE_URL` is set to
- if `None`, then default is ` "https://api.mistral.com/v1"`


- [x] **Add tests and docs**:

Added unit tests, docs I feel are unnecessary, as this is just aligning
with other packages that do the same?


- [x] **Lint and test**: 

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, ccurme, vbarda, hwchase17.

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/partners/mistralai/tests/unit_tests/test_chat_models.py

@@ -44,6 +44,39 @@ def test_mistralai_initialization() -> None:
         assert cast(SecretStr, model.mistral_api_key).get_secret_value() == "test"
 
 
+@pytest.mark.parametrize(
+    "model,expected_url",
+    [
+        (ChatMistralAI(model="test"), "https://api.mistral.ai/v1"),  # type: ignore[call-arg, arg-type]
+        (ChatMistralAI(model="test", endpoint="baz"), "baz"),  # type: ignore[call-arg, arg-type]
+    ],
+)
+def test_mistralai_initialization_baseurl(
+    model: ChatMistralAI, expected_url: str
+) -> None:
+    """Test ChatMistralAI initialization."""
+    # Verify that ChatMistralAI can be initialized providing endpoint, but also
+    # with default
+
+    assert model.endpoint == expected_url
+
+
+@pytest.mark.parametrize(
+    "env_var_name",
+    [
+        ("MISTRAL_BASE_URL"),
+    ],
+)
+def test_mistralai_initialization_baseurl_env(env_var_name: str) -> None:
+    """Test ChatMistralAI initialization."""
+    # Verify that ChatMistralAI can be initialized using env variable
+    import os
+
+    os.environ[env_var_name] = "boo"
+    model = ChatMistralAI(model="test")  # type: ignore[call-arg]
+    assert model.endpoint == "boo"
+
+
 @pytest.mark.parametrize(
     ("message", "expected"),
     [