Set a default region to Workhorse S3 client

By default, fog-aws uses "us-east-1" as the region, but the AWS SDK
does not. Transfers will fail if a region is not supplied:

```
MissingRegion: could not find region configuration
```

This commit sends Fog's default region (us-east-1) to ensure files get
uploaded properly.

changelogs/unreleased/sh-workhorse-default-region.yml

+---
+title: Set a default region to Workhorse S3 client
+merge_request: 59932
+author:
+type: fixed

lib/object_storage/direct_upload.rb

@@ -79,7 +79,7 @@ module ObjectStorage
           Provider: 'AWS',
           S3Config: {
             Bucket: bucket_name,
-            Region: credentials[:region],
+            Region: credentials[:region] || ::Fog::AWS::Storage::DEFAULT_REGION,
             Endpoint: credentials[:endpoint],
             PathStyle: config.use_path_style?,
             UseIamProfile: config.use_iam_profile?,

spec/lib/object_storage/direct_upload_spec.rb

@@ -126,6 +126,16 @@ RSpec.describe ObjectStorage::DirectUpload do
         expect(s3_config.keys).not_to include(%i(ServerSideEncryption SSEKMSKeyID))
       end
 
+      context 'when no region is specified' do
+        before do
+          raw_config.delete(:region)
+        end
+
+        it 'defaults to us-east-1' do
+          expect(subject[:ObjectStorage][:S3Config][:Region]).to eq('us-east-1')
+        end
+      end
+
       context 'when feature flag is disabled' do
         before do
           stub_feature_flags(use_workhorse_s3_client: false)