Fix PDF thumbnail generation – add System.Drawing reference and reset stream position (#40023)

##  Problem

[PdfThumbnailProvider](cci:1://file:///d:/Github/PowerToys/src/modules/previewpane/PdfThumbnailProvider/PdfThumbnailProvider.cs:15:8-18:9)
could fail at compile-time **and/or** produce empty or invalid
thumbnails at runtime:

1.
[Image](cci:1://file:///d:/Github/PowerToys/src/modules/previewpane/PdfThumbnailProvider/PdfThumbnailProvider.cs:81:8-103:9)
/ `Bitmap` types come from **System.Drawing**, but the namespace was
missing, causing build errors in environments that don’t rely on
implicit global usings.
2. After rendering the first PDF page to an in-memory stream, the
stream’s cursor remained at the end.
`Image.FromStream(stream.AsStream())` therefore read **zero bytes**,
throwing an exception or creating an empty bitmap.

##  Fix
* Added explicit `using System.Drawing;`.
* Added `stream.Seek(0);` right before `Image.FromStream(...)` to rewind
the stream to the beginning.

```csharp
using System.Drawing;   // new
...
page.RenderToStreamAsync(...).GetAwaiter().GetResult();
stream.Seek(0);         // rewind so Image.FromStream reads data
imageOfPage = Image.FromStream(stream.AsStream());
```
## Testing

- Rebuilt the solution – compilation succeeds without missing-namespace
errors.
- Ran thumbnail handler against multiple PDFs; thumbnails now render
correctly and no runtime exceptions are thrown.

## Impact
Users regain reliable PDF thumbnail previews in Explorer, improving
usability and preventing crashes or blank icons.

---------

Co-authored-by: leileizhang <leilzh@microsoft.com>

src/modules/previewpane/PdfThumbnailProvider/PdfThumbnailProvider.cs

@@ -1,6 +1,7 @@
-// Copyright (c) Microsoft Corporation
+// Copyright (c) Microsoft Corporation
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
+using System.Drawing;
 using Windows.Data.Pdf;
 using Windows.Storage;
 using Windows.Storage.Streams;
@@ -95,6 +96,8 @@ namespace Microsoft.PowerToys.ThumbnailHandler.Pdf
                 DestinationHeight = height,
             }).GetAwaiter().GetResult();
 
+            stream.Seek(0);
+
             imageOfPage = Image.FromStream(stream.AsStream());
 
             return imageOfPage;