Allow disabling `structuredContent` duplication in McpAsyncServer

首先阅读 McpAsyncServer.StructuredOutputCallToolHandler 及其 apply() 入口点，重点关注 structuredContent 被复制到 content 的位置。确定适当的配置机制，并验证用户可以启用或禁用重复，同时保留现有的默认行为。

Expected Behavior

Make io.modelcontextprotocol.server.McpAsyncServer.StructuredOutputCallToolHandler configurable to allow users to decide whether they want to duplicate the structuredContent in the content property.

This configuration could be introduced via a bean or another appropriate mechanism.

A possible solution implementation might look like this:

@Override
public Mono<CallToolResult> apply(McpAsyncServerExchange exchange, McpSchema.CallToolRequest request) {

    return this.delegateCallToolResult.apply(exchange, request).map(result -> {

        // Skipping some code for brevity

        if (Utils.isEmpty(result.content()) && keepCompatibility()) {  // <-- HERE keepCompatibility() METHOD IS NEW
            // For backwards compatibility, a tool that returns structured
            // content SHOULD also return functionally equivalent unstructured
            // content. (For example, serialized JSON can be returned in a
            // TextContent block.)
            // https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content

            return CallToolResult.builder()
                .content(List.of(new McpSchema.TextContent(validation.jsonStructuredOutput())))
                .isError(result.isError())
                .structuredContent(result.structuredContent())
                .build();
            }

            return result;
    });
}

private boolean keepCompatibility() {
    // Provide a way for users to specify whether to keep compatibility or not
}

Current Behavior

Currently, StructuredOutputCallToolHandler always duplicates structuredContent into the content property. This behavior is not mandated by the MCP specification.

Context

MCP tool users may be dissatisfied receiving duplicate content because:

• It causes unnecessary network traffic.
• It reduces performance.
• Most importantly, it consumes unnecessary space in the LLM context window on the client side.

There is no current way to prevent StructuredOutputCallToolHandler#apply() from copying structuredContent into content because the StructuredOutputCallToolHandler class is private and cannot be extended or customized externally.