WindowEvaluator already provides the pure-Python window UDF base class #1577 (item 6) asks for #1670

Split out from #1577 (item 6), which asks for a WindowUDF ABC equivalent to Accumulator.

Checked this against current main: datafusion.user_defined.WindowEvaluator already provides the capability. crates/core/src/udwf.rs::RustPartitionEvaluator dispatches memoize / get_range / evaluate / evaluate_all / evaluate_all_with_rank / supports_bounded_execution / uses_window_frame / include_rank via call_method, and python/tests/test_udwf.py has several pure-Python subclasses driven through udwf().

It's deliberately not metaclass=ABCMeta like Accumulator, though: which evaluate* method must be implemented depends on the uses_window_frame / supports_bounded_execution / include_rank flag matrix documented on the class, so no single method can be marked @abstractmethod the way Accumulator's can.

Opening this to confirm: is the literal "ABC" form still wanted on top of what exists (and if so, what should the abstractness contract look like given the flag matrix), or should #1577 drop this item?