fix: Group chats to include costs from all agents

[marklysze]

Why are these changes needed?

Fixed response.cost returning 0 in a group chat by recalculating cost to include ALL agents.

The issue was that initiate_chat() only gathered usage from [last_agent, manager], missing other participating agents.

Added cost recalculation in both initiate_group_chat() and a_initiate_group_chat() using gather_usage_summary() with all agents from the group chat manager's groupchat.agents.

Please note that this does not do nested cost calculations, only the agents associated with the top-level group chat.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/latest/docs/contributor-guide/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.

[joggrbot]

📝 Documentation Analysis

All docs are up to date! 🎉

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[priyansh4320]

Thank you @marklysze, this is great.

[SirEntropy]

LGTM!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
autogen/agentchat/group/multi_agent_chat.py	39.81% <100.00%> (+8.74%)	⬆️

... and 22 files with indirect coverage changes

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