Enable ToolResult Type in Core/AgentChat

[victordibia]

Currently, in Core/AgentChat the result of tool execution is mostly cast to a string ...
It would be good to standardize on some type e.g,

• content
• attachments
• mime
• byte ..

Several benefits

• We can cast strucutred responses into messages that other agents/teams can "see" .e.g, inform them of artifacts created from tool call.
• Interfaces like AGS can display such artifacts

How

• Add toolresult type
• Update core infra for tool call execution to check for structured output and honor them
• Update things like AssistantAgent to parse structured output into inner_messages etc?

[ekzhu]

Yes, this should be added, similar to the structured message type #5131. Though I would consider instead of going for mime type we can stay in Python land for now.

When we type hint a function tool, the return type can be a BaseModel. Thus it is possible to capture the result as part of the structured tool result message, with type information embedded in the message itself.

@jackgerrits some ideas on this? This will enable many scenarios.