together, standard-tests: specify tool_choice in standard tests (#25548)

Here we allow standard tests to specify a value for `tool_choice` via a
`tool_choice_value` property, which defaults to None.

Chat models [available in
Together](https://docs.together.ai/docs/chat-models) have issues passing
standard tool calling tests:
- llama 3.1 models currently [appear to rely on user-side
parsing](https://docs.together.ai/docs/llama-3-function-calling) in
Together;
- Mixtral-8x7B and Mistral-7B (currently tested) consistently do not
call tools in some tests.

Specifying tool_choice also lets us remove an existing `xfail` and use a
smaller model in Groq tests.

libs/partners/mistralai/tests/integration_tests/test_standard.py

@@ -1,6 +1,6 @@
 """Standard LangChain interface tests"""
 
-from typing import Type
+from typing import Optional, Type
 
 from langchain_core.language_models import BaseChatModel
 from langchain_standard_tests.integration_tests import (  # type: ignore[import-not-found]
@@ -18,3 +18,8 @@ class TestMistralStandard(ChatModelIntegrationTests):
     @property
     def chat_model_params(self) -> dict:
         return {"model": "mistral-large-latest", "temperature": 0}
+
+    @property
+    def tool_choice_value(self) -> Optional[str]:
+        """Value to use for tool choice when used in tests."""
+        return "any"