Spring AI ClassCastException: MessageDeltaEvent cannot be cast to ToolUseAggregationEvent in StreamHelper.mergeToolUseEvents when using Anthropic streaming with tools

## Bug Description When using Spring AI with Anthropic Claude and MCP tools in streaming mode, a ClassCastException occurs preventing tool execution.

## Stack Trace java.lang.ClassCastException: class org.springframework.ai.anthropic.api.AnthropicApi$MessageDeltaEvent cannot be cast to class org.springframework.ai.anthropic.api.AnthropicApi$ToolUseAggregationEvent at org.springframework.ai.anthropic.api.StreamHelper.mergeToolUseEvents(StreamHelper.java:86) at reactor.core.publisher.MonoReduceSeed$ReduceSeedSubscriber.onNext(MonoReduceSeed.java:116) ...

## Environment - Spring AI: 1.0.2, 1.1.0-M1 (both affected) - Spring Boot: 3.5.6 - MCP SDK: 0.13.0 - Tools: MCP Gmail integration

## Reproduction 1. Configure Anthropic ChatClient with MCP tools 2. Call .stream() on ChatClient with tool-enabled prompt 3. ClassCastException occurs when tool execution is attempted

## Expected Behavior Tools should execute without ClassCastException during streaming

## Actual Behavior ClassCastException prevents all tool execution in streaming mode

## Workaround None found. Switching to .call() (non-streaming) might work but defeats the purpose.