Fix potential stream hangs by ensuring completion callback (#52)

## Issue
Closes langchain4j/langchain4j#2048

## Change
Ensure that the completion callback is called even if the
[DONE] event is not received.

## General checklist
- [X] There are no breaking changes
- [ ] I have added unit and/or integration tests for my change
- [ ] The tests cover both positive and negative cases
- [X] I have manually run all the unit and integration tests in the
module I have added/changed, and they are all green

Author: emredmrcan

src/main/java/dev/ai4j/openai4j/StreamingRequestExecutor.java

@@ -167,7 +167,6 @@ public void onEvent(EventSource eventSource, String id, String type, String data
                 }
 
                 if ("[DONE]".equals(data)) {
-                    streamingCompletionCallback.run();
                     return;
                 }
 
@@ -192,6 +191,8 @@ public void onClosed(EventSource eventSource) {
                 if (logStreamingResponses) {
                     log.debug("onClosed()");
                 }
+
+                streamingCompletionCallback.run();
             }
 
             @Override