`permutedims(::RectDiagonal, perm)` isn't `RectDiagonal`

For example:

julia> using FillArrays: RectDiagonal

julia> d = RectDiagonal(randn(4))
4×4 RectDiagonal{Float64, Vector{Float64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}}:
 -2.47027   ⋅          ⋅        ⋅ 
   ⋅       0.139025    ⋅        ⋅ 
   ⋅        ⋅        -1.05975   ⋅ 
   ⋅        ⋅          ⋅       0.41858

julia> permutedims(d, (2, 1))
4×4 Matrix{Float64}:
 -2.47027  0.0        0.0      0.0
  0.0      0.139025   0.0      0.0
  0.0      0.0       -1.05975  0.0
  0.0      0.0        0.0      0.41858

julia> pkgversion(FillArrays)
v"1.13.0"

julia> versioninfo()
Julia Version 1.11.6
Commit 9615af0f269 (2025-07-09 12:58 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 10 × Apple M1 Max
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)

Reproduce the Julia example with RectDiagonal and permutedims(d, (2, 1)), then trace the permutedims behavior for RectDiagonal. Add a regression test for the demonstrated case and verify that the result remains a RectDiagonal with the same diagonal values rather than a Matrix.