Improve AnthropicChatModel handling of empty generations

 - Add handling for cases where ChatCompletion has a stop reason but empty generations.
   Creates a Generation with empty content and metadata when this occurs.
 - Update AnthropicChatModelObservationIT to expect "end_turn" finish reason.
 - Update javadoc.

Author: Claudio-code

models/spring-ai-anthropic/src/main/java/org/springframework/ai/anthropic/AnthropicChatModel.java

@@ -70,6 +70,7 @@
  * @author luocongqiu
  * @author Mariusz Bernacki
  * @author Thomas Vitale
+ * @author Claudio Silva Junior
  * @since 1.0.0
  */
 public class AnthropicChatModel extends AbstractToolCallSupport implements ChatModel {
@@ -293,6 +294,12 @@ private ChatResponse toChatResponse(ChatCompletionResponse chatCompletion) {
 
 		List<Generation> allGenerations = new ArrayList<>(generations);
 
+		if (chatCompletion.stopReason() != null && generations.isEmpty()) {
+			Generation generation = new Generation(new AssistantMessage(null, Map.of()),
+					ChatGenerationMetadata.from(chatCompletion.stopReason(), null));
+			allGenerations.add(generation);
+		}
+
 		List<ContentBlock> toolToUseList = chatCompletion.content()
 			.stream()
 			.filter(c -> c.type() == ContentBlock.Type.TOOL_USE)

models/spring-ai-anthropic/src/test/java/org/springframework/ai/anthropic/AnthropicChatModelObservationIT.java

@@ -118,7 +118,7 @@ void observationForStreamingChatOperation() {
 		ChatResponseMetadata responseMetadata = lastChatResponse.getMetadata();
 		assertThat(responseMetadata).isNotNull();
 
-		validate(responseMetadata, KeyValue.NONE_VALUE);
+		validate(responseMetadata, "[\"end_turn\"]");
 	}
 
 	private void validate(ChatResponseMetadata responseMetadata, String finishReasons) {