Set _isExperimental. Add comment to describe the property.

Author: mdaigle

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlConnection.cs

@@ -155,6 +155,7 @@ public SqlRetryLogicBaseProvider RetryLogicProvider
 
         internal SqlConnection(string connectionString, bool useExperimental) : this()
         {
+            _isExperimental = useExperimental;
             if (_isExperimental)
             {
                 _sqlConnectionX = new SqlClientX.SqlConnectionX(connectionString);
@@ -169,6 +170,7 @@ public SqlConnection(string connectionString) : this(connectionString, false)
 
         internal SqlConnection(string connectionString, SqlCredential credential, bool useExperimental) : this()
         {
+            _isExperimental = useExperimental;
             if (_isExperimental)
             {
                 _sqlConnectionX = new SqlClientX.SqlConnectionX(connectionString, credential);

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClient/SqlConnectionHelper.cs

@@ -22,6 +22,10 @@ public sealed partial class SqlConnection : DbConnection
         private DbConnectionOptions _userConnectionOptions;
         private DbConnectionPoolGroup _poolGroup;
         private DbConnectionInternal _innerConnection;
+        /// <summary>
+        /// Indicates that this connection uses the experimental SqlClientX code path.
+        /// When true, operations pass through _sqlConnectionX.
+        /// </summary>
         private bool _isExperimental;
         private SqlConnectionX _sqlConnectionX;
         private int _closeCount;

src/Microsoft.Data.SqlClient/netcore/src/Microsoft/Data/SqlClientX/SqlConnectionX.cs

@@ -97,7 +97,6 @@ public override bool CanCreateBatch
         protected override DbProviderFactory? DbProviderFactory
             => throw new NotImplementedException();
 
-
         /// <inheritdoc/>
         public override void ChangeDatabase(string databaseName)
             => throw new NotImplementedException();