Broader mkl support

[jtravs]

Add wider support for MKL usage, specifically for dims > 2.

[Copilot]

Pull request overview

This PR broadens MKL support by adding a native MKL DFTI-based planning/execution path so that FFTs over subset dimensions (e.g., transforming one dimension of a 3D+ array) work under the "mkl" provider, and updates tests to validate these behaviors under MKL.

Changes:

• Added a new MKL backend (src/mkl_dfti.jl) implementing FFT/rFFT planning and execution via MKL’s DFTI API.
• Guarded FFTW-only functionality in src/fft.jl (wisdom + FFTW plan implementations) so MKL can supply its own implementations.
• Updated tests to run previously FFTW-only subset-dimension and backend conformance tests under MKL as well.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
test/runtests.jl	Removes FFTW-only gating and adds/adjusts tests to validate subset-dimension FFTs and AbstractFFTs backend tests under MKL.
src/mkl_dfti.jl	New MKL DFTI plan types and implementations for complex FFTs and real FFTs (rfft/brfft) including plan inversion and adjoint styles.
src/fft.jl	Makes wisdom a no-op under MKL and compiles FFTW-only plan implementations only for the "fftw" provider.
src/FFTW.jl	Conditionally includes mkl_dfti.jl when the provider is "mkl".

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[codecov]

Codecov Report

❌ Patch coverage is 79.84496% with 78 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.99%. Comparing base (566f3c6) to head (5588281).

Files with missing lines	Patch %	Lines
src/mkl_dfti.jl	80.67%	74 Missing ⚠️
src/fft.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   73.64%   75.99%   +2.34%     
==========================================
  Files           5        6       +1     
  Lines         535      933     +398     
==========================================
+ Hits          394      709     +315     
- Misses        141      224      +83     

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[jtravs]

@stevengj would this PR be interesting, or should I create a new MKLFFT.jl package? I would find this functionality useful in some circumstances, but it does break on Mac (was it ever really well supported?).