Fix 400 errors not being logged in multipart middleware

In https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5194, we
saw that Workhorse was logging 400 errors, but we could not see an
associated error in the API logs to explain why this was happening.

According to
https://github.com/rack/rack/blob/master/SPEC.rdoc#label-The+Body, Rack
v2.1+ now requires:

```
The Body must respond to `each` and must only yield String values
```

Since the response returned returned a plain string, we would only see
this mysterious error in the Puma stderr logs:

```
Read: #<NoMethodError: undefined method `each' for <String:0x1234>
```

Making things worse, Puma didn't display the backtrace, and this
exception wasn't caught by Sentry because the exception was happening
outside of the middleware chain.

We fix this by wrapping the string in an array.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/336609

Changelog: fixed

lib/gitlab/middleware/multipart.rb

@@ -177,7 +177,7 @@ module Gitlab
           @app.call(env)
         end
       rescue UploadedFile::InvalidPathError => e
-        [400, { 'Content-Type' => 'text/plain' }, e.message]
+        [400, { 'Content-Type' => 'text/plain' }, [e.message]]
       end
     end
   end

spec/lib/gitlab/middleware/multipart_spec.rb

@@ -77,7 +77,8 @@ RSpec.describe Gitlab::Middleware::Multipart do
             result = subject
 
             expect(result[0]).to eq(400)
-            expect(result[2]).to include('insecure path used')
+            expect(result[2]).to be_a(Array)
+            expect(result[2].first).to include('insecure path used')
           end
         end
       end