Implement consistent structuredContent handling per specification clarification

Summary

Following the specification issue modelcontextprotocol/specification#1411, the Python SDK needs to ensure consistent handling of structuredContent vs content fields in tool responses.

Problem

Currently there's inconsistency across MCP implementations:

• Some clients use structuredContent when available (e.g., Cursor)
• Others ignore it entirely
• This leads to different behavior with the same servers

Current SDK Behavior

The Python SDK needs to be reviewed to ensure it properly handles the relationship between structuredContent and content fields.

Required Changes

Once modelcontextprotocol/specification#1411 is resolved, update the SDK to:

1. Implement the specified client behavior for structuredContent
2. Ensure proper fallback to content when needed
3. Validate that servers populate both fields consistently
4. Add tests to verify the correct behavior
5. Ensure consistency with the TypeScript SDK implementation

References

• Specification issue: modelcontextprotocol/specification#1411
• TypeScript SDK issue: modelcontextprotocol/typescript-sdk#911
• Discord discussion: https://discord.com/channels/1358869848138059966/1411057080902746293

cc: @dsp-ant

首先阅读规范 issue modelcontextprotocol/specification#1411 以及链接的 TypeScript SDK issue；它们定义了此 Python SDK 必须匹配的行为。然后定位 Python 客户端和服务器的响应处理，并为已明确的 structuredContent 行为添加测试。完成的标准是实现指定的回退逻辑、验证服务器字段的一致性，并与 TypeScript SDK 保持一致。