Fix TableFormula error message to show existing Y value

[Copilot]

When adding a point with a duplicate X coordinate but different Y value, TableFormula.AddPoint showed the attempted Y value in the error message instead of the existing Y value.

Example:

tableFormula.AddPoint(0, 0);   // Add initial point
tableFormula.AddPoint(0, 1);   // Attempt to add duplicate X
// Before: "A point for x=0 was already added with y=1"  ← wrong, shows new value
// After:  "A point for x=0 was already added with y=0"  ← correct, shows existing value

Changes:

• TableFormula.cs: Pass existingPoint instead of point to the existing exception constructor
• TableFormulaSpecs.cs: Add test verifying error message contains existing Y value

Original prompt

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This section details on the original issue you should resolve

<issue_title>TableFormula: misleading error message when adding a point with existing x-value</issue_title>
<issue_description>If I try to add a point to the table formula in R, and the point with the same x (but a different y) value already exists in the table, I get the message A point for x=xValue was already added with y=newYValue:

tableParam$formula
TableFormula: 
  isTable: TRUE
  XDimension: Time
  UseDerivedValues: FALSE
  x= 0, y= 0, restartSolver= FALSE
  x= 60, y= 1, restartSolver= FALSE
  x= 120, y= 2, restartSolver= FALSE
  x= 180, y= 3, restartSolver= FALSE

tableParam$formula$addPoints(c(0, 1, 2), c(1, 5, 6))
Error in rClr::clrCall(self$ref, "AddPoint", xValues[i], yValues[i]) : 
  Type:    OSPSuite.Core.Domain.ValuePointAlreadyExistsForPointException
Message: A point for x=0 was already added with y=1
Method:  Int32 AddPoint(OSPSuite.Core.Domain.Formulas.ValuePoint)
Stack trace:
   bei OSPSuite.Core.Domain.Formulas.TableFormula.AddPoint(ValuePoint point)
   bei OSPSuite.Core.Domain.Formulas.TableFormula.AddPoint(Double x, Double y)

However this is wrong, as the point for x=1 was added with y = 0 and not y = 1.</issue_description>

<agent_instructions>The error is thrown in OSPSuite.Core.Domain.Formulas.TableFormula.AddPoint and must be changed there</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes TableFormula: misleading error message when adding a point with existing x-value #2777

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[rwmcintosh]

@copilot just call the existing constructor with existingPoint. Remove the additional constructor that was added.

[Copilot]

Done in commit 2d5ce04. Removed the new constructor overload and now calling the existing constructor with existingPoint instead.