Tiny improvement of StringBuilder.Append(StringBuilder) (#101020)

* Update StringBuilder.cs

* Add an assert to validate `ExpandByABlock` working as expected

src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs

@@ -846,7 +846,8 @@ private StringBuilder AppendCore(StringBuilder value, int startIndex, int count)
                 if (length == 0)
                 {
                     ExpandByABlock(count);
-                    length = Math.Min(m_ChunkChars.Length - m_ChunkLength, count);
+                    Debug.Assert(m_ChunkLength == 0 && m_ChunkChars.Length >= count);
+                    length = count;
                 }
                 value.CopyTo(startIndex, new Span<char>(m_ChunkChars, m_ChunkLength, length), length);