feat: refactor the POST agent/messages API to take multiple messages

[cpacker]

• POST /agents/messages now hits server.send_messages which accepts a list properly
• POST /agents/messages now accepts types List[Message] in addition to List[MessageCreate]
• MessageCreate has been limited to only accepting roles user and system, because it's too complex to allow the user to create other roles on the POST (might as well just require them to create Messages instead)
  • for example, user wants to send an assistant message in the list that has a message "hey how's it going chad", but they don't realize that they can't put that in content, but need to put it inside tool_calls as the kwarg to a send_message function, etc...

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