Improve overflow detection algorithm in XML documentation examples

[Copilot]

The XML documentation examples in include-tag.cs contained a flawed overflow detection algorithm that only caught the specific case where one parameter equals int.MaxValue and the other is positive. This approach missed many overflow scenarios like (int.MaxValue - 5) + 10.

Changes Made

Integer Add Method

Replaced the manual overflow check with .NET's built-in checked context:

// Before: Limited manual check
if ((a == int.MaxValue && b > 0) || (b == int.MaxValue && a > 0))
    throw new System.OverflowException();
return a + b;

// After: Comprehensive overflow detection
try
{
    return checked(a + b);
}
catch (OverflowException)
{
    throw new System.OverflowException();
}

Double Add Method

Enhanced floating-point overflow detection with proper infinity checks:

// Added comprehensive checks for infinity and overflow conditions
if (double.IsInfinity(a) || double.IsInfinity(b) || 
    (a == double.MaxValue && b > 0) || (b == double.MaxValue && a > 0))
    throw new System.OverflowException();

double result = a + b;
if (double.IsInfinity(result))
    throw new System.OverflowException();

Benefits

• More accurate: Uses .NET's comprehensive overflow detection instead of incomplete manual checks
• Educational value: Demonstrates proper .NET overflow handling techniques using checked context
• Maintains documentation focus: All XML documentation elements remain intact, preserving the example's primary purpose of demonstrating <include> tags and exception documentation
• Simpler: Less custom logic, leverages framework capabilities

The changes align with maintainer feedback suggesting the use of checked(a + b) while keeping the example focused on XML documentation features rather than complex mathematical algorithms.

Fixes #41596.

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