Implement GetOptimalObjectiveValue function to evaluate objective at solution point

[Copilot]

Overview

This PR implements the GetOptimalObjectiveValue function as requested in issue #25. The function evaluates the objective function of an optimization problem at a given solution point by leveraging the existing FindValueOfExpression function.

Implementation

The new function has the following signature:

func GetOptimalObjectiveValue(sol Solution) (float64, error)

The function works by:

1. Retrieving the optimization problem from the solution using GetProblem()
2. Extracting the objective expression from problem.Objective.Expression
3. Using the existing FindValueOfExpression function to substitute all variables in the objective expression with their values from the solution
4. Returning the computed objective value

Example Usage

// Create an optimization problem
p := problem.NewProblem("Example")
x := p.AddVariable()
y := p.AddVariable()

// Set objective: minimize 5*x + 3*y
obj := x.Multiply(symbolic.K(5.0)).Plus(y.Multiply(symbolic.K(3.0)))
p.SetObjective(obj, problem.SenseMinimize)

// Create a solution where x=2, y=4
sol := solution.DummySolution{
    Values: map[uint64]float64{
        x.ID: 2.0,
        y.ID: 4.0,
    },
    Status: solution_status.OPTIMAL,
    Problem: p,
}

// Calculate the objective value at this solution point
objValue, err := solution.GetOptimalObjectiveValue(&sol)
// Returns: 22.0 (which is 5*2 + 3*4)

Error Handling

The function includes proper error handling for:

• Solutions without an associated problem
• Problems without a defined objective
• Variables missing from the solution (propagated from FindValueOfExpression)

Testing

Added four comprehensive test cases covering:

• Simple linear objectives with multiple variables
• Constant objective functions
• Complex expressions with multiple operations
• Error cases (nil problem reference)

All existing tests continue to pass, confirming no regressions were introduced.

Closes #25

Original prompt

Can you attempt to create the GetOptimalObjectiveValue function by using the FindValueOfExpression function that is currently built for solution.Solution interfaces? Your solution should follow the instructions and signature listed in issue #25.

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[codecov]

Codecov Report

❌ Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.33%. Comparing base (0214315) to head (3023d9b).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
solution/solution.go	64.70%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   86.76%   87.33%   +0.57%     
==========================================
  Files          27       35       +8     
  Lines        3687     4185     +498     
==========================================
+ Hits         3199     3655     +456     
- Misses        440      474      +34     
- Partials       48       56       +8     

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