[DFT][rocFFT] Add regression tests for stride validation and rank handling

[zjin-lcf]

Summary

• move rocFFT stride validation out of commit.cpp into src/dft/backends/rocfft/stride_validation.hpp, a standard-library-only helper with no behavior change, so it can be tested without rocFFT or an AMD GPU
• add hardware-independent tests for the [DFT][rocFFT] Fix out-of-bounds stride handling #760 fixes: real forward strides that only fit the backward domain are rejected, INPUT/OUTPUT strides are validated per direction, and overlap caused by a non-unit innermost stride is rejected
• add rocFFT backend commit tests (gated on ONEMATH_ENABLE_ROCFFT_BACKEND, skipped without an AMD GPU) that assert the commit result directly, including a rank-4 descriptor throwing oneapi::math::unimplemented

Fixes #771

Test plan

• New tests on AMD Instinct MI100: 9 passed in both test_main_dft_ct and test_main_dft_rt
• In a cuFFT-only build (no rocFFT), the 5 hardware-independent tests compile and pass
• Reverting each [DFT][rocFFT] Fix out-of-bounds stride handling #760 fix makes the new tests fail:
  • innermost stride multiplier: both RejectsOverlapFromNonUnitInnermostStride tests fail
  • forward/backward direction validity: RejectsRealFwdStridesThatOnlyFitBackwardDomain (both) and RealIoStridesValidOnlyForForward fail
  • rank guard: RocfftCommit.RejectsRank4AsUnimplemented fails
• Full rocFFT DFT suite on MI100 gives identical results with and without this change
• Run the pre-commit clang-format hook