Add OpenAI provider unit test code

tests/providers/test_openai_provider.py

@@ -0,0 +1,46 @@
+import pytest
+from unittest.mock import MagicMock, patch
+from aisuite.providers.openai_provider import OpenaiProvider
+
+
+@pytest.fixture(autouse=True)
+def set_api_key_env(monkeypatch):
+    """Fixture to set the OPENAI_API_KEY environment variable."""
+    monkeypatch.setenv("OPENAI_API_KEY", "test-key")
+
+
+def test_openai_provider_chat_completions_create():
+    """
+    Test: Validate that OpenaiProvider.chat_completions_create correctly calls
+    client.chat.completions.create via the provider instance.
+    """
+    message_history = [{"role": "user", "content": "Hello"}]
+    model = "openai-gpt-4o"
+    temperature = 0.7
+
+    expected_response = {"choices": [{"message": {"content": "Hello back!"}}]}
+
+    provider = OpenaiProvider()
+
+    fake_client = MagicMock()
+    provider.client = fake_client
+    fake_chat = MagicMock()
+    fake_client.chat = fake_chat
+    fake_chat.completions = MagicMock()
+    fake_chat.completions.create.return_value = expected_response
+
+    with patch.object(
+        fake_chat.completions, "create", return_value=expected_response
+    ) as mock_create:
+        response = provider.chat_completions_create(
+            model=model,
+            messages=message_history,
+            temperature=temperature,
+        )
+
+        mock_create.assert_called_once_with(
+            model=model,
+            messages=provider.transformer.convert_request(message_history),
+            temperature=temperature,
+        )
+        assert response == expected_response