[ty] Better error message for **kwargs with non-mapping type in overloaded functions #2653
Description
As a follow-up to astral-sh/ruff#22805 (comment), the validation that double-starred keyword arguments are mappings should happen in builder.rs instead of the binding stage. This way, the error is reported before overload resolution, so users see "must be a mapping type" instead of a generic "no matching overload" error.