fix: preserve stream parameter in CompletionRequest (#47)

### Problem
The `stream` parameter was omitted from requests, causing unexpected
streaming behavior.

### Fix
Ensured the `stream` parameter is preserved in CompletionRequest and
ChatCompletionRequest.

### Tests
Added unit tests to verify that the `stream` parameter is respected for
both `true` and `false` values.

Closes #46

Author: keyboardsamurai

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

@@ -132,7 +132,7 @@ public DefaultOpenAiClient build() {
     @Override
     public SyncOrAsyncOrStreaming<CompletionResponse> completion(OpenAiClientContext context,
         CompletionRequest request) {
-        CompletionRequest syncRequest = CompletionRequest.builder().from(request).stream(null).build();
+        CompletionRequest syncRequest = CompletionRequest.builder().from(request).stream(false).build();
 
         return new RequestExecutor<>(
                 openAiApi.completions(context.headers(), syncRequest, apiVersion),
@@ -150,7 +150,7 @@ public SyncOrAsyncOrStreaming<CompletionResponse> completion(OpenAiClientContext
     public SyncOrAsyncOrStreaming<String> completion(OpenAiClientContext context, String prompt) {
         CompletionRequest request = CompletionRequest.builder().prompt(prompt).build();
 
-        CompletionRequest syncRequest = CompletionRequest.builder().from(request).stream(null).build();
+        CompletionRequest syncRequest = CompletionRequest.builder().from(request).stream(false).build();
 
         return new RequestExecutor<>(
                 openAiApi.completions(context.headers(), syncRequest, apiVersion),
@@ -167,7 +167,7 @@ public SyncOrAsyncOrStreaming<String> completion(OpenAiClientContext context, St
     @Override
     public SyncOrAsyncOrStreaming<ChatCompletionResponse> chatCompletion(OpenAiClientContext context,
         ChatCompletionRequest request) {
-        ChatCompletionRequest syncRequest = ChatCompletionRequest.builder().from(request).stream(null).build();
+        ChatCompletionRequest syncRequest = ChatCompletionRequest.builder().from(request).stream(false).build();
 
         return new RequestExecutor<>(
                 openAiApi.chatCompletions(context.headers(), syncRequest, apiVersion),
@@ -185,7 +185,7 @@ public SyncOrAsyncOrStreaming<ChatCompletionResponse> chatCompletion(OpenAiClien
     public SyncOrAsyncOrStreaming<String> chatCompletion(OpenAiClientContext context, String userMessage) {
         ChatCompletionRequest request = ChatCompletionRequest.builder().addUserMessage(userMessage).build();
 
-        ChatCompletionRequest syncRequest = ChatCompletionRequest.builder().from(request).stream(null).build();
+        ChatCompletionRequest syncRequest = ChatCompletionRequest.builder().from(request).stream(false).build();
 
         return new RequestExecutor<>(
                 openAiApi.chatCompletions(context.headers(), syncRequest, apiVersion),