Fixing problem with invalid parse scientific form of numbers. (dotnet#17296)

Author: Maximys

src/libraries/Common/src/System/Globalization/FormatProvider.Number.cs

@@ -517,14 +517,6 @@ private static unsafe bool ParseNumber(ref char* str, char* strEnd, NumberStyles
                             {
                                 exp = exp * 10 + (ch - '0');
                                 ch = ++p < strEnd ? *p : '\0';
-                                if (exp > 1000)
-                                {
-                                    exp = 9999;
-                                    while (ch >= '0' && ch <= '9')
-                                    {
-                                        ch = ++p < strEnd ? *p : '\0';
-                                    }
-                                }
                             } while (ch >= '0' && ch <= '9');
                             if (negExp)
                             {

src/libraries/System.Runtime.Numerics/tests/BigInteger/BigIntegerToStringTests.cs

@@ -439,6 +439,25 @@ public static void CustomFormatPerMille()
             RunCustomFormatToStringTests(s_random, "#\u2030000000", CultureInfo.CurrentCulture.NumberFormat.NegativeSign, 6, PerMilleSymbolFormatter);
         }
 
+        public static IEnumerable<object[]> RunFormatScientificNotationToBigIntegerAndViceVersaData()
+        {
+            yield return new object[] { "1E+1000", "1E+1000" };
+            yield return new object[] { "1E+1001", "1E+1001" };
+        }
+
+        [Theory]
+        [MemberData(nameof(RunFormatScientificNotationToBigIntegerAndViceVersaData))]
+        public static void RunFormatScientificNotationToBigIntegerAndViceVersa(string testingValue, string expectedResult)
+        {
+            BigInteger parsedValue;
+            string actualResult;
+
+            parsedValue = BigInteger.Parse(testingValue, NumberStyles.AllowExponent);
+            actualResult = parsedValue.ToString("E0");
+
+            Assert.Equal(expectedResult, actualResult);
+        }
+
         private static void RunSimpleProviderToStringTests(Random random, string format, NumberFormatInfo provider, int precision, StringFormatter formatter)
         {
             string test;