reflect_on_max_turns as an option in SelectorGroupChat

[erigrus]

Currently, when a group chat reaches the max_turns limit, the conversation abruptly ends without providing a summary of what occurred during the interaction. When e.g. the user is in control after this cutoff the last message when max_turns was hit may not be really helpful. This can be particularly disruptive in nested chats, where maintaining context is crucial.

To address this, I propose implementing a reflect_on_max_turns option (similar to the reflect_on_tool_use of agents) for group chats. This should automatically generate a summary when the turn limit is reached and add this as a message before terminating the chat, ensuring a smoother handover back to the user.

[ekzhu]

I think it's a good idea to introduce optional summarization, which can be independent from how the team was terminated.

Related #6160