Prior to this change, when we got a `CallToolResult` from an MCP server,
we JSON-serialized its `content` field as the `content` to send back to
the model as part of the function call output that we send back to the
model. This meant that we were dropping the `structuredContent` on the
floor.
Though reading
https://modelcontextprotocol.io/specification/2025-06-18/schema#tool, it
appears that if `outputSchema` is specified, then `structuredContent`
should be set, which seems to be a "higher-fidelity" response to the
function call. This PR updates our handling of `CallToolResult` to
prefer using the JSON-serialization of `structuredContent`, if present,
using `content` as a fallback.
Also, it appears that the sense of `success` was inverted prior to this
PR!
Michael Bolin
8ba8089592