Fix BigInteger UTF-8 parsing with Ukrainian culture NBSP handling

[Copilot]

Description

BigInteger parsing fails for UTF-8 byte arrays with Ukrainian culture (uk-UA) when using NumberStyles.AllowThousands. Ukrainian uses NBSP (U+00A0) as NumberGroupSeparator, but users type regular space (U+0020).

The MatchChars function in Number.Parsing.Common.cs had incomplete space equivalence logic:

• Handled NBSP→space direction only for char (UTF-16)
• Missing space→NBSP direction
• No UTF-8 multi-byte sequence support (NBSP = 0xC2 0xA0, narrow NBSP = 0xE2 0x80 0xAF)

Changes

Core parsing (Number.Parsing.Common.cs):

• Added bidirectional space-replacing character equivalence for char path
• Implemented UTF-8 multi-byte handling for NBSP (0xC2 0xA0) and narrow NBSP (0xE2 0x80 0xAF)
• Updated comments to reference all cultures using space-replacing separators

Testing (parse.ukUA.cs):

• Added tests validating UTF-8/string parsing parity with Ukrainian culture
• Covers trailing spaces and NBSP character handling

Example

var ukCulture = new CultureInfo("uk-UA");
CultureInfo.CurrentCulture = ukCulture;

// Previously failed, now works
byte[] utf8 = Encoding.UTF8.GetBytes("123 ");  // trailing space
var result = BigInteger.Parse(utf8, NumberStyles.AllowTrailingWhite);  // ✓

// NBSP also works
byte[] utf8Nbsp = Encoding.UTF8.GetBytes("1\u00a0234\u00a0567");
result = BigInteger.Parse(utf8Nbsp, NumberStyles.AllowThousands);  // ✓

Original prompt

---

This section details on the original issue you should resolve

<issue_title>System.Numerics.Tests.parseTest.RunParseToStringTests(culture: uk-UA) test failures</issue_title>
<issue_description>This test runs as part of the libraries outer loop.

Hit in #119427. Log file: https://helixr1107v0xdcypoyl9e7f.blob.core.windows.net/dotnet-runtime-refs-pull-119427-merge-e179d39d041e4c21b3/System.Runtime.Numerics.Tests/1/console.8967a987.log?skoid=8eda00af-b5ec-4be9-b69b-0919a2338892&sktid=72f988bf-86f1-41af-91ab-2d7cd011db47&skt=2025-10-01T01%3A05%3A30Z&ske=2025-10-01T02%3A05%3A30Z&sks=b&skv=2024-11-04&sv=2024-11-04&st=2025-10-01T01%3A05%3A30Z&se=2025-10-01T02%3A05%3A30Z&sr=c&sp=r&sig=7IW6R8WrZP0cNbaHbzQaVlxd8lGUkdAbHB4%2BpInETpk%3D

    System.Numerics.Tests.parseTest.RunParseToStringTests(culture: uk-UA) [FAIL]
      System.FormatException : The value could not be parsed.
      Stack Trace:
        /_/src/libraries/System.Runtime.Numerics/src/System/Number.BigInteger.cs(29,0): at System.Number.ThrowOverflowOrFormatException(ParsingStatus status)
        /_/src/libraries/System.Runtime.Numerics/src/System/Number.BigInteger.cs(136,0): at System.Number.ParseBigInteger[TChar](ReadOnlySpan`1 value, NumberStyles style, NumberFormatInfo info)
        /_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs(707,0): at System.Numerics.BigInteger.Parse(ReadOnlySpan`1 utf8Text, NumberStyles style, IFormatProvider provider)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(828,0): at System.Numerics.Tests.parseTest.VerifyParseSpanToString(String num1, NumberStyles ns, Boolean failureNotExpected, String expected)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(884,0): at System.Numerics.Tests.parseTest.VerifyParseToString(String num1, NumberStyles ns, Boolean failureNotExpected, String expected)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(818,0): at System.Numerics.Tests.parseTest.VerifyParseToString(String num1, NumberStyles ns, Boolean failureNotExpected)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(689,0): at System.Numerics.Tests.parseTest.VerifyNumberStyles(NumberStyles ns, Random random)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(52,0): at System.Numerics.Tests.parseTest.RunParseToStringTests(CultureInfo culture)
           at InvokeStub_parseTest.RunParseToStringTests(Object, Span`1)
        /_/src/libraries/System.Private.CoreLib/src/System/Reflection/MethodBaseInvoker.cs(95,0): at System.Reflection.MethodBaseInvoker.InvokeWithOneArg(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
    System.Numerics.Tests.parseTestThreshold.RunParseToStringTests(culture: uk-UA) [FAIL]
      System.FormatException : The value could not be parsed.
      Stack Trace:
        /_/src/libraries/System.Runtime.Numerics/src/System/Number.BigInteger.cs(29,0): at System.Number.ThrowOverflowOrFormatException(ParsingStatus status)
        /_/src/libraries/System.Runtime.Numerics/src/System/Number.BigInteger.cs(136,0): at System.Number.ParseBigInteger[TChar](ReadOnlySpan`1 value, NumberStyles style, NumberFormatInfo info)
        /_/src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs(707,0): at System.Numerics.BigInteger.Parse(ReadOnlySpan`1 utf8Text, NumberStyles style, IFormatProvider provider)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(828,0): at System.Numerics.Tests.parseTest.VerifyParseSpanToString(String num1, NumberStyles ns, Boolean failureNotExpected, String expected)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(884,0): at System.Numerics.Tests.parseTest.VerifyParseToString(String num1, NumberStyles ns, Boolean failureNotExpected, String expected)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(818,0): at System.Numerics.Tests.parseTest.VerifyParseToString(String num1, NumberStyles ns, Boolean failureNotExpected)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(689,0): at System.Numerics.Tests.parseTest.VerifyNumberStyles(NumberStyles ns, Random random)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(52,0): at System.Numerics.Tests.parseTest.RunParseToStringTests(CultureInfo culture)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(1349,0): at System.Numerics.Tests.parseTestThreshold.<>c__DisplayClass2_0.<RunParseToStringTests>b__1()
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/BigIntTools.cs(54,0): at BigIntTools.Utils.RunWithFakeThreshold(Int32& field, Int32 value, Action action)
        /_/src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs(1347,0): at System.Numerics.Tests.parseTestThreshold.<>c__DisplayClass2_0.<RunParseToStringTests>b__0()
        /_/src/libraries/System.Runtime....

</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes dotnet/runtime#120283

<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more [Copilot coding agent tips](https://gh.io/copilot-coding-agent-tips) in the docs.