Move custom ActiveStorage service to $LOAD_PATH

We moved this file to `app/lib/` in commit cb477149c so it would be in
the autoload_paths. However, this class is loaded by ActiveStorage, with
the following method:

```
def resolve(class_name)
  require "active_storage/service/#{class_name.to_s.underscore}_service"
  ActiveStorage::Service.const_get(:"#{class_name.camelize}Service")
rescue LoadError
  raise "Missing service adapter for #{class_name.inspect}"
end
``

So this file needs to be in the $LOAD_PATH, or else ActiveStorage won't
be able to load it when we disable the `add_autoload_paths_to_load_path`
option, which is the default in Rails 7.1 [1].

Moving it to the `lib` folder solves the issue; as mentioned in the
guide to upgrade to Rails 7.1 [2]:

> The lib directory is not affected by this flag, it is added to
> $LOAD_PATH always.

However, we were also referencing this class in the `Tenant` model,
meaning we needed to autoload it as well somehow. So, instead of
directly referencing this class, we're using `respond_to?` in the Tenant
model.

We're changing the test so it fails when the code calls
`is_a?(ActiveStorage::Service::TenantDiskService)`. We need to change
the active storage configurations in the test because, otherwise, the
moment `ActiveStorage::Blob` is loaded, the `TenantDiskService` class is
also loaded, meaning the test will pass when using `is_a?`.

Note that, since this class isn't in the autoload paths anymore, we need
to add a `require` in the tests. We could add an initializer to require
it; we're not doing it in order to be consistent with what ActiveStorage
does: it only loads the service that's going to be used in the current
Rails environment. If somebody changed their production environment in
order to use (for example), S3, and we added an initializer to require
the TenantDiskService, we would still load the TenantDiskService even if
it isn't going to be used.

[1] https://guides.rubyonrails.org/v7.1/configuring.html#config-add-autoload-paths-to-load-path
[2] https://guides.rubyonrails.org/v7.1/upgrading_ruby_on_rails.html#autoloaded-paths-are-no-longer-in-$load-path

spec/lib/active_storage/service/tenant_disk_service_spec.rb

@@ -1,4 +1,5 @@
 require "rails_helper"
+require "active_storage/service/tenant_disk_service"
 
 describe ActiveStorage::Service::TenantDiskService do
   describe "#tenant_root_for" do