Add non-ASCII XML streaming tests for SqlDataReader.GetChars

Add tests that verify SqlDataReader.GetChars correctly handles non-ASCII
characters when reading XML columns with SequentialAccess:

- GetChars_NonAsciiContent: Latin accented characters (2-byte UTF-8)
- GetChars_NonAsciiContent_BulkRead: Bulk read path with accented chars
- GetChars_CjkContent: CJK characters (3-byte UTF-8)

These tests establish a baseline for correct behavior on main before
PR #3974 (issue #1877) refactors SqlStreamingXml internals.

Author: mdaigle

src/Microsoft.Data.SqlClient/tests/ManualTests/Microsoft.Data.SqlClient.ManualTests.csproj

@@ -85,6 +85,7 @@
     <Compile Include="SQL\DataReaderTest\DataReaderStreamsTest.cs" />
     <Compile Include="SQL\DataReaderTest\DataReaderTest.cs" />
     <Compile Include="SQL\DataStreamTest\DataStreamTest.cs" />
+    <Compile Include="SQL\SqlStreamingXmlTest\SqlStreamingXmlTest.cs" />
     <Compile Include="SQL\DateTimeTest\DateTimeTest.cs" />
     <Compile Include="SQL\DNSCachingTest\DNSCachingTest.cs" />
     <Compile Include="SQL\ExceptionTest\ConnectionExceptionTest.cs" />

src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/SqlStreamingXmlTest/SqlStreamingXmlTest.cs

@@ -0,0 +1,117 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Data;
+using Xunit;
+
+namespace Microsoft.Data.SqlClient.ManualTesting.Tests
+{
+    public static class SqlStreamingXmlTest
+    {
+        [ConditionalFact(typeof(DataTestUtility), nameof(DataTestUtility.AreConnStringsSetup))]
+        public static void GetChars_NonAsciiContent()
+        {
+            SqlConnection connection = new(DataTestUtility.TCPConnectionString);
+            // XML containing non-ASCII characters:
+            //   - \u00E9 (e-acute) - 2 bytes in UTF-8
+            //   - \u00F1 (n-tilde) - 2 bytes in UTF-8
+            //   - \u00FC (u-umlaut) - 2 bytes in UTF-8
+            string xml = "<r>caf\u00E9 se\u00F1or \u00FCber</r>";
+            int expectedLength = xml.Length;
+            string commandText = $"SELECT Convert(xml, N'{xml}')";
+
+            using (SqlCommand command = connection.CreateCommand())
+            {
+                connection.Open();
+                command.CommandText = commandText;
+
+                SqlDataReader sqlDataReader = command.ExecuteReader(CommandBehavior.SequentialAccess);
+                Assert.True(sqlDataReader.Read(), "Expected to read a row");
+
+                (long length, string result) = ReadAllChars(sqlDataReader, expectedLength + 10);
+
+                Assert.Equal(expectedLength, length);
+                Assert.Equal(xml, result.Substring(0, (int)length));
+                connection.Close();
+            }
+        }
+
+        [ConditionalFact(typeof(DataTestUtility), nameof(DataTestUtility.AreConnStringsSetup))]
+        public static void GetChars_NonAsciiContent_BulkRead()
+        {
+            SqlConnection connection = new(DataTestUtility.TCPConnectionString);
+            // Same non-ASCII XML but read in a single bulk GetChars call
+            string xml = "<name>Jos\u00E9 Garc\u00EDa</name>";
+            int expectedLength = xml.Length;
+            string commandText = $"SELECT Convert(xml, N'{xml}')";
+
+            using (SqlCommand command = connection.CreateCommand())
+            {
+                connection.Open();
+                command.CommandText = commandText;
+
+                SqlDataReader sqlDataReader = command.ExecuteReader(CommandBehavior.SequentialAccess);
+                Assert.True(sqlDataReader.Read(), "Expected to read a row");
+
+                char[] buffer = new char[expectedLength + 10];
+                long charsRead = sqlDataReader.GetChars(0, 0, buffer, 0, buffer.Length);
+
+                Assert.Equal(expectedLength, charsRead);
+                string result = new string(buffer, 0, (int)charsRead);
+                Assert.Equal(xml, result);
+                connection.Close();
+            }
+        }
+
+        [ConditionalFact(typeof(DataTestUtility), nameof(DataTestUtility.AreConnStringsSetup))]
+        public static void GetChars_CjkContent()
+        {
+            SqlConnection connection = new(DataTestUtility.TCPConnectionString);
+            // CJK characters: 3 bytes each in UTF-8
+            string xml = "<data>\u65E5\u672C\u8A9E\u30C6\u30B9\u30C8</data>";
+            int expectedLength = xml.Length;
+            string commandText = $"SELECT Convert(xml, N'{xml}')";
+
+            using (SqlCommand command = connection.CreateCommand())
+            {
+                connection.Open();
+                command.CommandText = commandText;
+
+                SqlDataReader sqlDataReader = command.ExecuteReader(CommandBehavior.SequentialAccess);
+                Assert.True(sqlDataReader.Read(), "Expected to read a row");
+
+                (long length, string result) = ReadAllChars(sqlDataReader, expectedLength + 10);
+
+                Assert.Equal(expectedLength, length);
+                Assert.Equal(xml, result.Substring(0, (int)length));
+                connection.Close();
+            }
+        }
+
+        /// <summary>
+        /// Read all chars one at a time using GetChars with SequentialAccess,
+        /// replicating the pattern from issue #1877.
+        /// </summary>
+        private static (long, string) ReadAllChars(SqlDataReader sqlDataReader, int expectedSize)
+        {
+            char[] text = new char[expectedSize];
+            char[] buffer = new char[1];
+
+            long position = 0;
+            long numCharsRead;
+            do
+            {
+                numCharsRead = sqlDataReader.GetChars(0, position, buffer, 0, 1);
+                if (numCharsRead > 0)
+                {
+                    text[position] = buffer[0];
+                    position += numCharsRead;
+                }
+            }
+            while (numCharsRead > 0);
+
+            return (position, new string(text));
+        }
+    }
+}