[FEATURE] UX: AI Service: when @MemoryId is used, validate that chatMemoryProvider is configured

[langchain4j]

When @MemoryId is used in AI Service, validate that chatMemoryProvider is configured.

public interface Assistant {

    String chat(@MemoryId String memoryId, @UserMessage String userMessage);
}

Currently it throws NPE:

java.lang.NullPointerException: Cannot invoke "dev.langchain4j.memory.chat.ChatMemoryProvider.get(Object)" because "this.chatMemoryProvider" is null
	at dev.langchain4j.service.AiServiceContext.lambda$chatMemory$1(AiServiceContext.java:47)
	at java.base/java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1708)
	at dev.langchain4j.service.AiServiceContext.chatMemory(AiServiceContext.java:47)
	at dev.langchain4j.service.DefaultAiServices$1.invoke(DefaultAiServices.java:105)