Fix Group Chat usage summary (tokens + cost)

[marklysze]

UPDATE: We are exploring the native integration of telemetry to provide a more holistic capture of the key events and costs of your AG2 workflow. So, this PR may be supeseded by that integration. Exploration continues...

Why are these changes needed?

An issue raised by @bassilkhilo, #103, identified a bug with the ChatResult of a group chat not including the costs.

After investigating, I've found that when calculating the costs in initiate_chat/a_initiate_chat it is not looking at the costs of the agents in the group chat (only the initiating agent and the group chat manager).

This addresses that.

The status of this PR is that it is pulling through the costs of the agents in the group chat, however it is not including the costs of the speaker selection agent when in auto speaker selection mode. This needs to be addressed as well.

As it is, this will work for a swarm, which doesn't use that.

TODO: Review nested chats to ensure those costs are coming through as well.

Related issue number

Closes #103

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.

[bassilkhilo]

Hey @marklysze This works great for non SwarmResult agent transfers!

However as you've mentioned, if one is using SwarmResult to transfer to another agent within a function, this wouldn't take that agent's cost into account since it's with the temporary AssistantAgent.

Great work! Looking forward to having this include the costs of SwarmResult transfers :) Would be epic

[bassilkhilo]

Also, @marklysze had a great solution on using initiate_chat with a dummy user proxy agent (max turns 1) to get the cost of any inner chat that would normally use generate_reply. This can be summed with the Swarm costs to get a total cost.