Make ProjectTo{AbstractArray} inferrable for wrappers with OneTo axes

[devmotion]

Fixes #407.

ProjectTo{AbstractArray} chooses between dx and reshape(dx, project.axes) based on the values of the axes. For wrappers like Diagonal, UpperTriangular or Symmetric, reshape returns a ReshapedArray, so the inferred result is a Union:

julia> @inferred ProjectTo(rand(3, 3))(Diagonal(rand(3)))
ERROR: return type Diagonal{Float64, Vector{Float64}} does not match inferred return type Union{Base.ReshapedArray{...}, Diagonal{Float64, Vector{Float64}}}

If dx and the projector have the same number of OneTo axes but they are not identical, the sizes differ and the size check throws anyway. This PR checks that case on types, so inference no longer sees the reshape. Behaviour is unchanged: the same inputs throw the same error, and reshape is still used for trivial dimensions and OffsetArrays.

• Adds regression tests for ProjectTo(::AbstractArray) does not infer #407.
• The Symmetric allocation test no longer needs to be marked broken.

Note: #702 changes the same lines; whichever is merged second needs to add !isempty(dx) to the new condition.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.40%. Comparing base (0da460d) to head (a7e3338).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #716      +/-   ##
==========================================
+ Coverage   93.39%   93.40%   +0.01%     
==========================================
  Files          15       15              
  Lines         999     1001       +2     
==========================================
+ Hits          933      935       +2     
  Misses         66       66              

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