Fix: Enable max_retries Parameter in ChatMistralAI Class (#30448)

**partners: Enable max_retries in ChatMistralAI**

**Description**

- This pull request reactivates the retry logic in the
completion_with_retry method of the ChatMistralAI class, restoring the
intended functionality of the previously ineffective max_retries
parameter. New unit test that mocks failed/successful retry calls and an
integration test to confirm end-to-end functionality.

**Issue**
- Closes #30362

**Dependencies**
- No additional dependencies required

Co-authored-by: andrasfe <andrasf94@gmail.com>

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

@@ -2,8 +2,9 @@
 
 import os
 from typing import Any, AsyncGenerator, Dict, Generator, List, cast
-from unittest.mock import patch
+from unittest.mock import MagicMock, patch
 
+import httpx
 import pytest
 from langchain_core.callbacks.base import BaseCallbackHandler
 from langchain_core.messages import (
@@ -270,3 +271,46 @@ def test_extra_kwargs() -> None:
     # Test that if provided twice it errors
     with pytest.raises(ValueError):
         ChatMistralAI(model="my-model", foo=3, model_kwargs={"foo": 2})  # type: ignore[call-arg]
+
+
+def test_retry_with_failure_then_success() -> None:
+    """Test that retry mechanism works correctly when
+    first request fails and second succeeds."""
+    # Create a real ChatMistralAI instance
+    chat = ChatMistralAI(max_retries=3)
+
+    # Set up the actual retry mechanism (not just mocking it)
+    # We'll track how many times the function is called
+    call_count = 0
+
+    def mock_post(*args: Any, **kwargs: Any) -> MagicMock:
+        nonlocal call_count
+        call_count += 1
+
+        if call_count == 1:
+            raise httpx.RequestError("Connection error", request=MagicMock())
+
+        mock_response = MagicMock()
+        mock_response.status_code = 200
+        mock_response.json.return_value = {
+            "choices": [
+                {
+                    "message": {
+                        "role": "assistant",
+                        "content": "Hello!",
+                    },
+                    "finish_reason": "stop",
+                }
+            ],
+            "usage": {
+                "prompt_tokens": 1,
+                "completion_tokens": 1,
+                "total_tokens": 2,
+            },
+        }
+        return mock_response
+
+    with patch.object(chat.client, "post", side_effect=mock_post):
+        result = chat.invoke("Hello")
+        assert result.content == "Hello!"
+        assert call_count == 2, f"Expected 2 calls, but got {call_count}"