Switched the Plus methods of all vector expression objects to take …

[kwesiRutledge]

…advantage of the VectorPlusTemplate + included test from gonum bug work

[codecov]

Codecov Report

❌ Patch coverage is 85.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.00%. Comparing base (5549451) to head (aac662c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
symbolic/monomial_vector.go	50.00%	2 Missing ⚠️
symbolic/constant_vector.go	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   87.09%   87.00%   -0.09%     
==========================================
  Files          42       42              
  Lines        6516     6394     -122     
==========================================
- Hits         5675     5563     -112     
+ Misses        743      734       -9     
+ Partials       98       97       -1     

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[Copilot]

Pull Request Overview

This PR refactors the Plus methods across multiple vector expression types to utilize a centralized VectorPlusTemplate function instead of duplicating addition logic. The change simplifies vector arithmetic operations by consolidating common patterns and includes a test case related to debugging a gonum issue.

• Replaced type-specific addition logic with calls to VectorPlusTemplate
• Added comprehensive test case for large matrix multiplication scenarios
• Improved error handling consistency across vector types

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
testing/symbolic/constant_matrix_test.go	Added test case for 57-variable matrix multiplication to verify gonum bug fix
symbolic/polynomial_vector.go	Simplified Plus method to use VectorPlusTemplate for all Expression types
symbolic/monomial_vector.go	Replaced complex type-specific logic with VectorPlusTemplate and added int support
symbolic/constant_vector.go	Refactored Plus method to use VectorPlusTemplate and improved error handling

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