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

Leggi prima l’issue della specifica modelcontextprotocol/specification#1411 e l’issue collegata del TypeScript SDK; definiscono il comportamento a cui questo Python SDK deve conformarsi. Poi individua la gestione delle risposte del client e del server Python e aggiungi test per il comportamento chiarito di structuredContent. Il lavoro è completato quando è stato implementato il fallback specificato, sono stati convalidati i campi coerenti del server e il risultato corrisponde al TypeScript SDK.