Add factorial calculation utility service

[Copilot]

Adds a general-purpose MathService to support factorial computation of non-negative integers.

New surface area

• IMathService — interface with long CalculateFactorial(int n)
• MathService — iterative implementation; supports inputs 0–20 (full long-safe range)
  • n < 0 → ArgumentOutOfRangeException
  • n > 20 → OverflowException (21! exceeds Int64.MaxValue)

Usage

IMathService math = new MathService();

math.CalculateFactorial(0);   // 1
math.CalculateFactorial(10);  // 3628800
math.CalculateFactorial(20);  // 2432902008176640000
math.CalculateFactorial(-1);  // throws ArgumentOutOfRangeException
math.CalculateFactorial(21);  // throws OverflowException

Tests

15 xUnit tests in MathServiceTests covering base cases (0, 1), a parameterised sweep of known values up to 20!, and both error paths.

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