Bare JSON scalars (0, true, null, "string") and arrays pass
json.Valid() but the Kiro API rejects them in the json field with
ValidationException. Once such a tool result is in session history,
every subsequent request fails, making the session unrecoverable.
Restrict the json path to content starting with '{' so only actual
JSON objects are sent that way; everything else uses the text field.
a035913514