fix: GroupChatManager async run throws an exception if no eligible speaker

[leryor]

Why are these changes needed?

The Autogen introduced an async implementation for the nested chats in v0.2.35, but it currently has an issue with finishing conversation. It rises an exception NoEligibleSpeaker as a signal that the nested chat has finished but it doesn't handle properly so the exception interrupts the whole agentic flow.

To fix the issue, the exception should be handled in GroupChatManager:a_run_chat() in the same way as the synchronous version does GroupChatManager:run_chat().

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/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.

[ilia-shutov-tfs]

Thank you @orlov332 for a fix! Without this fix, we can't use autogen in asynchronous way (From Python code perspective)