Refactor ComposeSignaturePreviewPng to return Stream

Updated the `ComposeSignaturePreviewPng` method in the `IPdfPageImageRenderer` interface to return a `Stream` instead of a `byte[]` for improved memory efficiency when handling large image data.

- Added `ComposeSignaturePreviewPngBytes` extension method to convert the `Stream` to a `byte[]` for backward compatibility.
- Updated `PdfPageImageRenderer` to return a `Stream` and adjusted its internal logic to reset the `MemoryStream` position before returning.
- Modified `DevSignatureInkController` to use the `Stream`-based implementation for HTTP responses.
- Made minor formatting adjustments in `PdfPageImageRenderer`.

These changes optimize memory usage while maintaining compatibility with existing workflows that require `byte[]`.

EnvelopeGenerator.Infrastructure.Doc/PdfPageImageRenderer.cs

@@ -57,7 +57,7 @@ public class PdfPageImageRenderer : IPdfPageImageRenderer
         return Task.FromResult<IReadOnlyList<PdfPageImageDto>>(results);
     }
 
-    public byte[] ComposeSignaturePreviewPng(ISignatureInkPayload element)
+    public Stream ComposeSignaturePreviewPng(ISignatureInkPayload element)
     {
         const double sigW = 1.77 * 72;
         const double sigH = 1.96 * 72;
@@ -137,9 +137,10 @@ public class PdfPageImageRenderer : IPdfPageImageRenderer
 
         var renderOptions = PdfPageRenderingParameters.CreateWithResolution(400f);
         using DXBitmap bitmap = rasterProcessor.CreateDXBitmap(1, renderOptions);
-        using var pngStream = new MemoryStream();
+        var pngStream = new MemoryStream();
         bitmap.Save(pngStream, DXImageFormat.Png);
-        return pngStream.ToArray();
+        pngStream.Position = 0;
+        return pngStream;
     }
 
     private static System.Drawing.RectangleF FitImageContain(System.Drawing.RectangleF area, byte[] imageBytes)
@@ -176,4 +177,4 @@ public class PdfPageImageRenderer : IPdfPageImageRenderer
         return isJpeg;
     }
 
-}
+}