Refactor: Add null check, optimize string joining, and add JavaDocs (#3663)

* Refactor: Add null check, optimize string joining, and add JavaDocs

Auto-cherry-pick to 1.0.x
Fixes #3663

Signed-off-by: nirsa <islandtim@naver.com>

Author: KoreaNirsa

models/spring-ai-openai/src/main/java/org/springframework/ai/openai/api/OpenAiApi.java

@@ -21,6 +21,7 @@
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Consumer;
 import java.util.function.Predicate;
+import java.util.stream.Collectors;
 
 import com.fasterxml.jackson.annotation.JsonFormat;
 import com.fasterxml.jackson.annotation.JsonIgnore;
@@ -144,11 +145,20 @@ public OpenAiApi(String baseUrl, ApiKey apiKey, MultiValueMap<String, String> he
 			.build(); // @formatter:on
 	}
 
+	/**
+	 * Returns a string containing all text values from the given media content list. Only
+	 * elements of type "text" are processed and concatenated in order.
+	 * @param content The list of {@link ChatCompletionMessage.MediaContent}
+	 * @return a string containing all text values from "text" type elements
+	 * @throws IllegalArgumentException if content is null
+	 */
 	public static String getTextContent(List<ChatCompletionMessage.MediaContent> content) {
+		Assert.notNull(content, "content cannot be null");
+
 		return content.stream()
 			.filter(c -> "text".equals(c.type()))
 			.map(ChatCompletionMessage.MediaContent::text)
-			.reduce("", (a, b) -> a + b);
+			.collect(Collectors.joining());
 	}
 
 	/**