Swarm - First Speaker

[marklysze]

Why are these changes needed?

Updates to the First Speaker handling for the swarm.

Changes to initiate_swarm_chat:

• Will create a temporary UserProxyAgent agent if: only one message passed in + the message doesn't have a name + no user agent has been passed in
• Will clear the temporary UserProxyAgent's name from the chat history before returning from initiate_swarm_chat
• init_agent parameter name changed to initial_agent for clarity

Related issue number

N/A

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[qingyun-wu]

Looks good! Thank you Mark!