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.Application/Common/Interfaces/Doc/IPdfPageImageRenderer.cs

@@ -22,7 +22,7 @@ public interface IPdfPageImageRenderer
     /// </summary>
     /// <param name="element"></param>
     /// <returns></returns>
-    byte[] ComposeSignaturePreviewPng(ISignatureInkPayload element);
+    Stream ComposeSignaturePreviewPng(ISignatureInkPayload element);
 
     /// <summary>
     /// 
@@ -51,4 +51,16 @@ public static class PdfPageImageRendererExtensions
 
         return inkBytes;
     }
-}
+
+    public static byte[] ComposeSignaturePreviewPngBytes(this IPdfPageImageRenderer renderer, ISignatureInkPayload element)
+    {
+        using var stream = renderer.ComposeSignaturePreviewPng(element);
+
+        if (stream is MemoryStream memoryStream)
+            return memoryStream.ToArray();
+
+        using var copy = new MemoryStream();
+        stream.CopyTo(copy);
+        return copy.ToArray();
+    }
+}