fix(agentchat): validate participants type in BaseGroupChat with clear TypeError

[xodn348]

Summary

Fixes #7580

When a user accidentally passes None, a non-list value (e.g. a string or integer), or a list containing non-agent/non-team objects as participants to RoundRobinGroupChat (or any BaseGroupChat subclass), the current code raises low-level internal errors that are hard to debug:

• TypeError: object of type 'NoneType' has no len() — for participants=None
• AttributeError: 'str' object has no attribute 'name' — for participants="not a list" or a list containing a string

Changes

• _base_group_chat.py: Added explicit type validation at the top of BaseGroupChat.__init__ — checks that participants is a list and that every element is a ChatAgent or Team instance, raising a descriptive TypeError before any internal logic runs.
• _round_robin_group_chat.py: Updated the Raises docstring to document the new TypeError.
• tests/test_group_chat.py: Added test_group_chat_invalid_participants — a synchronous test covering all four bad-input cases: None, a string, an integer, and a list containing a non-agent item.

Before / After

# Before (confusing internal error)
RoundRobinGroupChat(participants=None)
# TypeError: object of type 'NoneType' has no len()

# After (clear, actionable message)
RoundRobinGroupChat(participants=None)
# TypeError: participants must be a list of ChatAgent or Team instances, got 'NoneType'.

# Before
RoundRobinGroupChat(participants=[agent, "bad"])
# AttributeError: 'str' object has no attribute 'name'

# After
RoundRobinGroupChat(participants=[agent, "bad"])
# TypeError: participants[1] must be a ChatAgent or Team instance, got 'str'.

The fix is in BaseGroupChat, so it covers all subclasses (RoundRobinGroupChat, SelectorGroupChat, Swarm, MagenticOneGroupChat) automatically.

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