fix(agentchat): raise clear errors for invalid group chat participants

[MukundaKatta]

Summary

• validate group chat participants before the runtime tries to read names or lengths from invalid inputs
• raise a clear TypeError when callers pass None, strings, or non-agent items
• add regression tests covering the invalid RoundRobinGroupChat inputs from the issue

Closes #7580

Verification

• python3 -m pytest python/packages/autogen-agentchat/tests/test_group_chat.py -k "rejects_non_sequence_participants or rejects_string_participants or rejects_non_agent_participants" (fails in this environment before collection because pytest_asyncio is unavailable)
• PYTHONPYCACHEPREFIX=/tmp/pycache python3 -m py_compile python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_base_group_chat.py python/packages/autogen-agentchat/tests/test_group_chat.py