fix(mobile): use originalFileName for multipart filename in background uploads

The iOS background upload path set the multipart file part `filename` to the
PhotoKit temp export basename (e.g.
`0EAAEF1B-...-_o_IMG_9237.MOV`) while the foreground path and the `filename`
form field both use the clean `originalFileName` (e.g. `IMG_9237.MOV`).

This made the same asset appear as two different uploads in raw HTTP/access
logs and broke reverse proxies / upload optimizers that use the multipart
filename as part of a dedup key.

Set the multipart filename to `originalFileName ?? filename` so both upload
paths and the form field agree. Covers regular and live-photo uploads since
both route through `buildUploadTask`.

Closes #28601

mobile/lib/services/background_upload.service.dart

@@ -422,7 +422,7 @@ class BackgroundUploadService {
       httpRequestMethod: 'POST',
       url: url,
       headers: headers,
-      filename: filename,
+      filename: originalFileName ?? filename,
       fields: fieldsMap,
       baseDirectory: baseDirectory,
       directory: directory,

mobile/test/services/background_upload.service_test.dart

@@ -82,6 +82,8 @@ void main() {
 
       expect(task, isNotNull);
       expect(task!.fields['filename'], equals('OriginalPhoto.jpg'));
+      // multipart file part filename must match the clean originalFileName (foreground/background parity)
+      expect(task.filename, equals('OriginalPhoto.jpg'));
       verify(() => mockAssetMediaRepository.getOriginalFilename(asset.id)).called(1);
     });
 
@@ -118,6 +120,7 @@ void main() {
       expect(task, isNotNull);
       // For live photos, extension should be changed to match the video file
       expect(task!.fields['filename'], equals('OriginalLivePhoto.mov'));
+      expect(task.filename, equals('OriginalLivePhoto.mov'));
       verify(() => mockAssetMediaRepository.getOriginalFilename(asset.id)).called(1);
     });
   });