This PR fixes an issue where XmlReader.Create(stream) throws an undocumented ArgumentOutOfRangeException instead of the expected XmlException when parsing malformed XML containing invalid UTF-8 sequences in the XML declaration.

Problem

When parsing XML like <?xml version="1.0\xbf"?> from a MemoryStream, the following exception was thrown:

System.ArgumentOutOfRangeException: count ('-2') must be a non-negative value. (Parameter 'count')
   at System.Buffer.BlockCopy(Array src, Int32 srcOffset, Array dst, Int32 dstOffset, Int32 count)
   at System.Xml.XmlTextReaderImpl.ReadData()

This is problematic because:

1. ArgumentOutOfRangeException is not documented for XmlReader methods
2. XML parsing errors should consistently throw XmlException
3. This makes it impossible to catch all XML-related errors with a single catch block

Root Cause

The issue occurs in XmlTextReaderImpl.ReadData() when:

1. UnDecodeChars() calculates _ps.bytePos using _ps.encoding.GetByteCount() for malformed UTF-8 sequences
2. The calculated _ps.bytePos becomes greater than _ps.bytesUsed due to encoding issues
3. bytesLeft = _ps.bytesUsed - _ps.bytePos becomes negative (-2)
4. This negative value is passed to Buffer.BlockCopy(), causing the exception

Solution

Added bounds checking in XmlTextReaderImpl.ReadData() to detect when bytesLeft is negative and throw an appropriate XmlException with the message "Invalid character in the given encoding" instead of allowing the negative value to reach Buffer.BlockCopy().

The fix is minimal and surgical - it only adds validation where the problem occurs without changing broader parsing logic.

Testing

Added a regression test ReadWithMalformedUtf8InXmlDeclaration() that verifies:

• The specific issue case no longer throws ArgumentOutOfRangeException
• An appropriate XmlException is thrown instead
• The exception message is correct

Fixes #113061.

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