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

首先閱讀規格 issue modelcontextprotocol/specification#1411 以及連結的 TypeScript SDK issue；它們定義了此 Python SDK 必須符合的行為。接著找出 Python 用戶端和伺服器的回應處理，並為已釐清的 structuredContent 行為新增測試。完成的標準是實作指定的 fallback、驗證伺服器欄位的一致性，並與 TypeScript SDK 保持一致。