Optimize ToFlatOpcString to reduce memory usage and improve performan… (#1863)

## Optimized the ToFlatOpcString method to address high memory usage and
improve performance.

The previous implementation relied on LINQ, which created a significant
amount of intermediate collections and dictionaries, leading to
inefficiency, especially when working with Word documents containing
images.

This update removes unnecessary LINQ operations and reduces memory
overhead, replace manual chunking logic with Convert.ToBase64String
using Base64FormattingOptions.InsertLineBreaks.

This approach eliminates the need for LINQ-based chunking and
aggregation, resulting in cleaner, more efficient code while adhering to
the MIME specification for line breaks in Base64 encoding.

A sample benchmark output when working with a 822KB-word document.


![image](https://github.com/user-attachments/assets/3efadb6b-9f0f-41d5-9e8d-9992d0296469)

Co-authored-by: Dat Hoang Quoc <[email protected]>
Co-authored-by: Taylor Southwick <[email protected]>

Author: 3 people

src/DocumentFormat.OpenXml.Framework/Packaging/FlatOpcExtensions.cs

@@ -141,25 +141,7 @@ private static string ToChunkedBase64String(IPackagePart part)
     }
 
     private static string ToChunkedBase64String(byte[] byteArray)
-    {
-        // The MIME specification defines a maximum line length of 76 characters
-        // for a Base64-encoded string. Therefore, we need to break down the
-        // Base64 string into chunks of up to 76 characters each.
-        const int maxLineLength = 76;
-
-        return Convert.ToBase64String(byteArray)
-            .Select((@char, index) => new { Character = @char, Chunk = index / maxLineLength })
-            .GroupBy(charAndChunk => charAndChunk.Chunk)
-            .Aggregate(
-                new StringBuilder(),
-                (sb, grouping) => sb
-                    .Append(grouping.Aggregate(
-                        new StringBuilder(),
-                        (chunkSb, charAndChunk) => chunkSb.Append(charAndChunk.Character),
-                        chunkSb => chunkSb.ToString()))
-                    .Append(Environment.NewLine),
-                sb => sb.ToString());
-    }
+        => Convert.ToBase64String(byteArray, Base64FormattingOptions.InsertLineBreaks);
 
     internal static IPackageFactory<TPackage> WithFlatOpcTemplate<TPackage>(this IPackageFactory<TPackage> builder, string text, bool? isEditable = default)
         where TPackage : OpenXmlPackage