Fix #7580 - Add TypeError validation for participants in BaseGroupChat.__init__

[Whning0513]

RoundRobinGroupChat raises raw AttributeError/TypeError for invalid participants instead of a clear validation error

When participants is accidentally set to None, a string, an integer, or a list containing a non-agent object, the RoundRobinGroupChat constructor raises low-level Python errors such as:

• TypeError: object of type 'NoneType' has no len()
• AttributeError: 'str' object has no attribute 'name'

These are realistic user mistakes that should produce clear validation errors instead of leaking internal implementation details.

Reported by: emberpath24

The BaseGroupChat.__init__ method (inherited by RoundRobinGroupChat) immediately attempted to call len(participants) and iterate over participants without validating the type or content of the participants argument first. This caused raw Python runtime errors that were confusing to users.

File: autogen/python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_base_group_chat.py

Added validation checks at the beginning of BaseGroupChat.__init__:

1. Empty participants check (line 79-80): Raises TypeError("At least one participant is required.") if len(participants) == 0
2. Type check per participant (lines 81-83): Iterates over all participants and raises TypeError(f"All participants must be ChatAgent instances, got {type(participant).__name__}") if any participant is not a ChatAgent instance

These checks ensure users receive clear, actionable error messages when providing invalid input.

Low - The change only adds validation at the top of __init__ before any other logic executes. It does not modify any existing behavior for valid inputs. The validation is minimal and defensive, matching standard Python practices for constructor argument validation.

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