Fix for Explicit returns mixed with implicit (fall through) returns

[Priyanka-Microsoft]

The best fix is to add an explicit return None statement at the end of the select_agent method. This will ensure the method always returns a value explicitly, even if no branch in the match is taken (e.g. if history[-1].name is an unexpected value). This change will clarify developer intention and satisfy CodeQL. Only src/backend/sql_agents/helpers/comms_manager.py around the select_agent method needs modification; specifically, add an explicit return None after the match statement.

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