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

Leia primeiro a issue da especificação modelcontextprotocol/specification#1411 e a issue vinculada do TypeScript SDK; elas definem o comportamento ao qual este Python SDK deve corresponder. Em seguida, localize o tratamento de respostas do cliente e do servidor Python e adicione testes para o comportamento esclarecido de structuredContent. A tarefa estará concluída quando o fallback especificado estiver implementado, os campos consistentes do servidor tiverem sido validados e o resultado corresponder ao TypeScript SDK.