ForeignAggregateUDF does not forward distinct handling

[mkleen]

Is your feature request related to a problem or challenge?

#25288 adds AggregateUDFImpl::distinct_handling (datafusion/expr/src/udaf.rs:958), which lets an
aggregate declare whether DISTINCT can change its result, and EliminateAggregateDistinct uses that
to drop the modifier from duplicate-insensitive functions.

ForeignAggregateUDF (datafusion/ffi/src/udaf/mod.rs:471) does not forward the new method, and
FFI_AggregateUDF (datafusion/ffi/src/udaf/mod.rs:59) has no field to carry it, so every UDAF loaded
over FFI falls back to the default DistinctHandling::Sensitive.

The effect is that the new API silently does nothing across the FFI boundary. A third-party min-like
or set-valued aggregate that declares DistinctHandling::Insensitive still gets the full
SingleDistinctToGroupBy treatment — a per-group hash set and an extra grouping stage that only
deduplicates input the function was going to ignore — while the identical function registered
in-process does not. Results stay correct either way; the optimization is just unreachable.

Describe the solution you'd like

Carry the tag through the vtable the way order_sensitivity already does:

1. Add FFI_DistinctHandling (#[repr(C)]) with From impls in both directions, alongside
   FFI_AggregateOrderSensitivity at datafusion/ffi/src/udaf/mod.rs:628.
2. Add a distinct_handling fn pointer to FFI_AggregateUDF and a distinct_handling_fn_wrapper,
   mirroring order_sensitivity_fn_wrapper (datafusion/ffi/src/udaf/mod.rs:328).
3. Implement distinct_handling on ForeignAggregateUDF as
   unsafe { (self.udaf.distinct_handling)(&self.udaf).into() }.
4. Add a round-trip test over every variant, like test_round_trip_all_order_sensitivities
   (datafusion/ffi/src/udaf/mod.rs:864), plus a ForeignAggregateUDF test asserting a foreign UDAF
   that declares Insensitive reports Insensitive.

One wrinkle worth deciding: DistinctHandling is #[non_exhaustive], so the From<FFI_DistinctHandling>
conversion needs a policy for a variant added by a newer library on the other side of the boundary.
Mapping anything unrecognized to Sensitive keeps it conservative — that is the default, and it only
ever costs an optimization, never correctness.

Describe alternatives you've considered

Leaving it as is. FFI aggregates keep working and return correct results; they just never benefit from
EliminateAggregateDistinct. The cost is that the two registration paths behave differently for the
same function, which is surprising and hard to notice.

[mkleen]

take