feat: implement getindex method for ArrowheadMatrix

[Nimrais]

1. ArrowheadMatrix:

   • Added getindex with one-time performance warning
   • Enhanced error handling for invalid indices
   • Implemented ≈ (isapprox) operator
   • Improved matrix multiplication consistency

2. InvArrowheadMatrix:

   • Implemented efficient dot(x, A, y) for real vectors
   • Utilized linsolve! to avoid explicit inverse computation

3. General Improvements:

   • Ensured error consistency with dense matrices
   • Expanded test suite for new features and edge cases
   • Optimized performance with in-place operations

These changes need to these PR's tests to pass ReactiveBayes/ExponentialFamily.jl#206

[bvdmitri]

Nice!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.22%. Comparing base (741a8c7) to head (c4c3ba0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   76.80%   77.22%   +0.41%     
==========================================
  Files          12       12              
  Lines        1194     1216      +22     
==========================================
+ Hits          917      939      +22     
  Misses        277      277              

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