Fix TryParsePartial under-reporting charsConsumed with leading whitespace

Co-authored-by: EgorBo <523221+EgorBo@users.noreply.github.com>

Author: Copilot

src/libraries/System.Private.CoreLib/src/System/Number.Parsing.cs

@@ -380,13 +380,14 @@ internal static ParsingStatus TryParseBinaryIntegerStyle<TChar, TInteger>(ReadOn
                 }
                 else
                 {
-                    value = value.Slice(index);
-                    index = 0;
+                    // Slice a copy rather than reassigning value, so that index (and thus the number
+                    // of elements reported as consumed) stays relative to the original input.
+                    ReadOnlySpan<TChar> remaining = value.Slice(index);
 
                     ReadOnlySpan<TChar> positiveSign = info.PositiveSignTChar<TChar>();
                     ReadOnlySpan<TChar> negativeSign = info.NegativeSignTChar<TChar>();
 
-                    if (!positiveSign.IsEmpty && value.StartsWith(positiveSign))
+                    if (!positiveSign.IsEmpty && remaining.StartsWith(positiveSign))
                     {
                         index += positiveSign.Length;
 
@@ -396,7 +397,7 @@ internal static ParsingStatus TryParseBinaryIntegerStyle<TChar, TInteger>(ReadOn
                         }
                         num = TChar.CastToUInt32(value[index]);
                     }
-                    else if (!negativeSign.IsEmpty && value.StartsWith(negativeSign))
+                    else if (!negativeSign.IsEmpty && remaining.StartsWith(negativeSign))
                     {
                         isNegative = true;
                         index += negativeSign.Length;

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Int32Tests.cs

@@ -1056,6 +1056,15 @@ public static IEnumerable<object[]> TryParsePartial_TestData()
 
             // Stop at null character
             yield return new object[] { "123\0abc", NumberStyles.Integer, null, 123, 4 };
+
+            // Leading whitespace is counted as consumed even when the signs aren't the invariant "+"/"-"
+            NumberFormatInfo nonInvariantSignFormat = new NumberFormatInfo() { NegativeSign = "\u2212" };
+            yield return new object[] { " 5", NumberStyles.Integer, nonInvariantSignFormat, 5, 2 };
+            yield return new object[] { "  123abc", NumberStyles.Integer, nonInvariantSignFormat, 123, 5 };
+            yield return new object[] { "  +123abc", NumberStyles.Integer, nonInvariantSignFormat, 123, 6 };
+            yield return new object[] { "  \u2212456xyz", NumberStyles.Integer, nonInvariantSignFormat, -456, 6 };
+            yield return new object[] { "  \u2212456", NumberStyles.Integer, nonInvariantSignFormat, -456, 6 };
+            yield return new object[] { "  123  abc", NumberStyles.Integer, nonInvariantSignFormat, 123, 7 };
         }
 
         [Theory]

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/UInt32Tests.cs

@@ -481,6 +481,12 @@ public static IEnumerable<object[]> TryParsePartial_TestData()
 
             // Valid number without trailing characters
             yield return new object[] { "123", NumberStyles.Integer, null, 123u, 3 };
+
+            // Leading whitespace is counted as consumed even when the signs aren't the invariant "+"/"-"
+            NumberFormatInfo nonInvariantSignFormat = new NumberFormatInfo() { NegativeSign = "\u2212" };
+            yield return new object[] { " 5", NumberStyles.Integer, nonInvariantSignFormat, 5u, 2 };
+            yield return new object[] { "  123abc", NumberStyles.Integer, nonInvariantSignFormat, 123u, 5 };
+            yield return new object[] { "  +123abc", NumberStyles.Integer, nonInvariantSignFormat, 123u, 6 };
         }
 
         [Theory]