[Unit tests] Add unit tests for CmdPal Calc (#40770)

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## Summary of the Pull Request
Move calculator tests from PowerToys Run to CmdPal
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- [x] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
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src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Calc/Helper/CalculateEngine.cs

@@ -108,7 +108,7 @@ public static class CalculateEngine
     ///   100000.9999999999 → "100001"
     ///   1234567890123.45  → "1234567890123.45"
     /// </summary>
-    private static decimal FormatMax15Digits(decimal value, CultureInfo cultureInfo)
+    public static decimal FormatMax15Digits(decimal value, CultureInfo cultureInfo)
     {
         var absValue = Math.Abs(value);
         var integerDigits = absValue >= 1 ? (int)Math.Floor(Math.Log10((double)absValue)) + 1 : 1;