Fix langchain4j#1853 (langchain4j#1874)

## Issue
Fixes langchain4j#1853

## Change
`AiMessage` created by `OpenAiStreamingResponseBuilder` should contain
both text and tool calls if LLM responded with both.

## General checklist
- [X] There are no breaking changes
- [ ] I have added unit and integration tests for my change
- [X] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green
- [x] I have manually run all the unit and integration tests in the
[core](https://github.com/langchain4j/langchain4j/tree/main/langchain4j-core)
and
[main](https://github.com/langchain4j/langchain4j/tree/main/langchain4j)
modules, and they are all green
- [ ] I have added/updated the
[documentation](https://github.com/langchain4j/langchain4j/tree/main/docs/docs)
- [ ] I have added an example in the [examples
repo](https://github.com/langchain4j/langchain4j-examples) (only for
"big" features)
- [ ] I have added/updated [Spring Boot
starter(s)](https://github.com/langchain4j/langchain4j-spring) (if
applicable)

langchain4j-open-ai/src/main/java/dev/langchain4j/model/openai/InternalOpenAiHelper.java

@@ -133,6 +133,7 @@ public static Message toOpenAiMessage(ChatMessage message) {
                     .collect(toList());
 
             return AssistantMessage.builder()
+                    .content(aiMessage.text())
                     .toolCalls(toolCalls)
                     .build();
         }

langchain4j-open-ai/src/main/java/dev/langchain4j/model/openai/OpenAiStreamingResponseBuilder.java

@@ -18,8 +18,10 @@
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 
+import static dev.langchain4j.internal.Utils.isNullOrBlank;
 import static dev.langchain4j.model.openai.InternalOpenAiHelper.finishReasonFrom;
 import static dev.langchain4j.model.openai.InternalOpenAiHelper.tokenUsageFrom;
+import static java.util.Collections.singletonList;
 import static java.util.stream.Collectors.toList;
 
 /**
@@ -142,23 +144,21 @@ public void append(CompletionResponse partialResponse) {
 
     public Response<AiMessage> build() {
 
-        String content = contentBuilder.toString();
-        if (!content.isEmpty()) {
-            return Response.from(
-                    AiMessage.from(content),
-                    tokenUsage,
-                    finishReason
-            );
-        }
+        String text = contentBuilder.toString();
 
         String toolName = toolNameBuilder.toString();
         if (!toolName.isEmpty()) {
             ToolExecutionRequest toolExecutionRequest = ToolExecutionRequest.builder()
                     .name(toolName)
                     .arguments(toolArgumentsBuilder.toString())
                     .build();
+
+            AiMessage aiMessage = isNullOrBlank(text) ?
+                    AiMessage.from(toolExecutionRequest) :
+                    AiMessage.from(text, singletonList(toolExecutionRequest));
+
             return Response.from(
-                    AiMessage.from(toolExecutionRequest),
+                    aiMessage,
                     tokenUsage,
                     finishReason
             );
@@ -172,8 +172,21 @@ public Response<AiMessage> build() {
                             .arguments(it.argumentsBuilder.toString())
                             .build())
                     .collect(toList());
+
+            AiMessage aiMessage = isNullOrBlank(text) ?
+                    AiMessage.from(toolExecutionRequests) :
+                    AiMessage.from(text, toolExecutionRequests);
+
+            return Response.from(
+                    aiMessage,
+                    tokenUsage,
+                    finishReason
+            );
+        }
+
+        if (!isNullOrBlank(text)) {
             return Response.from(
-                    AiMessage.from(toolExecutionRequests),
+                    AiMessage.from(text),
                     tokenUsage,
                     finishReason
             );