Document how to use Mistral AI over Spring AI OpenAI API.

 - Add a related integration test: MistralWithOpenAiChatModelIT

Resolves #1134

Author: tzolov

models/spring-ai-mistral-ai/src/test/java/org/springframework/ai/mistralai/MistralAiChatModelIT.java

@@ -188,7 +188,8 @@ void beanStreamOutputConverterRecords() {
 	@Test
 	void functionCallTest() {
 
-		UserMessage userMessage = new UserMessage("What's the weather like in San Francisco? Response in Celsius");
+		UserMessage userMessage = new UserMessage(
+				"What's the weather like in San Francisco, Tokyo, and Paris? Response in Celsius");
 
 		List<Message> messages = new ArrayList<>(List.of(userMessage));