Remove legacy reduction SUM mode after hipDNN compatibility window

ReductionAttr::Mode::SUM was kept as a legacy compatibility spelling after ReductionAttr::Mode::ADD was added. ADD is the spelling aligned with cudnn-frontend and hipDNN, and SUM should only remain until hipDNN has consumed a Fusilli bump that no longer needs the legacy mode.

TODO reference: include/fusilli/attributes/reduction_attributes.h

Work items:
- Confirm hipDNN has bumped to a Fusilli revision that supports ReductionAttr::Mode::ADD.
- Replace remaining Fusilli references to ReductionAttr::Mode::SUM with ReductionAttr::Mode::ADD where appropriate.
- Remove SUM from FUSILLI_REDUCTION_MODES and any compatibility tests/docs that only cover the legacy spelling.
- Keep ADD lowering/test coverage intact, including the reduction emitter path that lowers ADD to sum semantics.

Acceptance criteria:
- Fusilli no longer exposes or internally uses ReductionAttr::Mode::SUM.
- ReductionAttr::Mode::ADD remains covered by tests and samples.
- The removal does not break the hipDNN integration after its Fusilli bump.