fix: GH-4472 1. Optimized the mergeToolUseEvents method in StreamHelper to enhance robustness when previousEvent is not a ToolUseAggregationEvent;

2. Fixed an issue in the `eventToChatCompletionResponse` method of `StreamHelper` where `thinkingBlock.signature()` was not being handled;
3. Improved `StreamHelperTests` to increase test coverage

Signed-off-by: Sun Yuhan <sunyuhan1998@users.noreply.github.com>

Author: sunyuhan1998

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023-2024 the original author or authors.
+ * Copyright 2023-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,13 +28,13 @@
 import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlock.Type;
 import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockDeltaEvent;
 import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockDeltaEvent.ContentBlockDeltaJson;
+import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockDeltaEvent.ContentBlockDeltaSignature;
 import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockDeltaEvent.ContentBlockDeltaText;
 import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockDeltaEvent.ContentBlockDeltaThinking;
-import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockDeltaEvent.ContentBlockDeltaSignature;
 import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockStartEvent;
 import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockStartEvent.ContentBlockText;
-import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockStartEvent.ContentBlockToolUse;
 import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockStartEvent.ContentBlockThinking;
+import org.springframework.ai.anthropic.api.AnthropicApi.ContentBlockStartEvent.ContentBlockToolUse;
 import org.springframework.ai.anthropic.api.AnthropicApi.EventType;
 import org.springframework.ai.anthropic.api.AnthropicApi.MessageDeltaEvent;
 import org.springframework.ai.anthropic.api.AnthropicApi.MessageStartEvent;
@@ -55,6 +55,9 @@
  * @author Christian Tzolov
  * @author Jihoon Kim
  * @author Alexandros Pappas
+ * @author Claudio Silva Junior
+ * @author Soby Chacko
+ * @author Sun Yuhan
  * @since 1.0.0
  */
 public class StreamHelper {
@@ -81,7 +84,9 @@ public boolean isToolUseFinish(StreamEvent event) {
 	 */
 	public StreamEvent mergeToolUseEvents(StreamEvent previousEvent, StreamEvent event) {
 
-		ToolUseAggregationEvent eventAggregator = (ToolUseAggregationEvent) previousEvent;
+		if (!(previousEvent instanceof ToolUseAggregationEvent eventAggregator)) {
+			return event;
+		}
 
 		if (event.type() == EventType.CONTENT_BLOCK_START) {
 			ContentBlockStartEvent contentBlockStart = (ContentBlockStartEvent) event;
@@ -159,7 +164,7 @@ else if (event.type().equals(EventType.CONTENT_BLOCK_START)) {
 			}
 			else if (contentBlockStartEvent.contentBlock() instanceof ContentBlockThinking thinkingBlock) {
 				ContentBlock cb = new ContentBlock(Type.THINKING, null, null, contentBlockStartEvent.index(), null,
-						null, null, null, null, null, thinkingBlock.thinking(), null);
+						null, null, null, null, thinkingBlock.signature(), thinkingBlock.thinking(), null);
 				contentBlockReference.get().withType(event.type().name()).withContent(List.of(cb));
 			}
 			else {