Update the AbstractMcpToolMethodCallback.convertValueToCallToolResult method

Hi! I suggest redesign this method because I think there are the following problems with this method:
1. The field ReturnMode.STRUCTURED is only for text content, so it shouldn't exist as a separate return form.
2. According to the MCP protocol For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block.



