Allow assistant messages in postRequestBodySchema for tool result handling

[ragingwind]

Description

The client will send a message includingt tool call results (with Human approval results) by addToolResult with received assistant's message before, but postRequestBodySchema.parse was rejecting them, causing validation errors in the chatbot SDK.

TO-BE

postRequestBodySchema.parse allows more schema for addToolResult

There are any reason to accept only user role mesage in chat route?