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

まず仕様 issue modelcontextprotocol/specification#1411 と、リンクされている TypeScript SDK issue を読み、それらがこの Python SDK が一致させるべき動作を定義していることを確認してください。次に、Python のクライアントおよびサーバーのレスポンス処理を特定し、明確化された structuredContent の動作に対するテストを追加してください。指定されたフォールバックを実装し、サーバーのフィールドの一貫性を検証し、TypeScript SDK と一致させれば完了です。