AssistantMessage supports reasoningContent

Expected Behavior
Currently, many model providers offer reason models, and users choose whether to use them depending on the specific use case. Spring AI needs to improve its support for reason models. add reasoningContent to AssistantMessage

public class AssistantMessage extends AbstractMessage implements MediaContent {

	private final List<ToolCall> toolCalls;

	protected final List<Media> media;
        private String reasoningContent 
        // other codes        
       ...

Current Behavior
At present, supporting reason models requires adapting to different model vendors. This makes it difficult to standardize and unify reasoning messages, and also results in relatively high adaptation costs.

Start by locating and reading the AssistantMessage class shown in the issue, including its existing content and tool-call fields. Trace how AssistantMessage instances are created and consumed, then consider the change complete when reasoningContent is represented consistently without disrupting existing message behavior.