API fix to System.Data batching API (#56525)

Allow implementations of DbBatchCommandCollection to hide the indexer
with a narrower-typed version for their own implementation of
DbBatchCommand, as elsewhere in ADO.NET.

Fixup for #54467, part of #28633

Author: roji

src/libraries/System.Data.Common/ref/System.Data.Common.cs

@@ -1945,10 +1945,10 @@ public abstract class DbBatch : System.IDisposable, System.IAsyncDisposable
     public abstract class DbBatchCommand
     {
         public abstract string CommandText { get; set; }
-        public abstract CommandType CommandType { get; set; }
+        public abstract System.Data.CommandType CommandType { get; set; }
         public abstract int RecordsAffected { get; }
-        public DbParameterCollection Parameters { get { throw null; } }
-        protected abstract DbParameterCollection DbParameterCollection { get; }
+        public System.Data.Common.DbParameterCollection Parameters { get { throw null; } }
+        protected abstract System.Data.Common.DbParameterCollection DbParameterCollection { get; }
     }
     public abstract class DbBatchCommandCollection : System.Collections.Generic.IList<DbBatchCommand>
     {
@@ -1964,7 +1964,9 @@ public abstract class DbBatchCommandCollection : System.Collections.Generic.ILis
         public abstract int IndexOf(DbBatchCommand item);
         public abstract void Insert(int index, DbBatchCommand item);
         public abstract void RemoveAt(int index);
-        public abstract DbBatchCommand this[int index] { get; set; }
+        public System.Data.Common.DbBatchCommand this[int index] { get { throw null; } set { throw null; } }
+        protected abstract System.Data.Common.DbBatchCommand GetBatchCommand(int index);
+        protected abstract void SetBatchCommand(int index, System.Data.Common.DbBatchCommand batchCommand);
     }
     public abstract partial class DbColumn
     {

src/libraries/System.Data.Common/src/System/Data/Common/DbBatchCommandCollection.cs

@@ -32,6 +32,14 @@ public abstract class DbBatchCommandCollection : IList<DbBatchCommand>
 
         public abstract void RemoveAt(int index);
 
-        public abstract DbBatchCommand this[int index] { get; set; }
+        public DbBatchCommand this[int index]
+        {
+            get => GetBatchCommand(index);
+            set => SetBatchCommand(index, value);
+        }
+
+        protected abstract DbBatchCommand GetBatchCommand(int index);
+
+        protected abstract void SetBatchCommand(int index, DbBatchCommand batchCommand);
     }
 }