Fix ProjectTo for symmetric sparse matrices.

[samuelsonric]

There PR fixes a bug in the ProjectTo function for Symmetric (and Hermitian)-wrapped sparse matrices.

julia> using ChainRulesCore, LinearAlgebra, SparseArrays
julia> A =Hermitian(triu(sprand(100, 100, .1)));
julia> ProjectTo(A)(A) == A
false

The comparison now correctly returns true.

[codecov]

Codecov Report

❌ Patch coverage is 88.50575% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.00%. Comparing base (dad0d24) to head (7730590).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/ChainRulesCoreSparseArraysExt.jl	88.50%	10 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #707 +/- ##
==========================================
- Coverage 93.40% 93.00% -0.40% 
==========================================
Files 15 15 Lines 1000 1087 +87 ==========================================
+ Hits 934 1011 +77 - Misses 66 76 +10 

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[devmotion]

LinearAlgebra is not available in the SparseArrays extension. Only ChainRulesCore and SparseArrays are.