Fix boot crash on Redmine 7.0 / Rails 8: assign model_object instead of using removed DSL

[BrutalBirdie]

Fixes #133

Problem

With Redmine 7.0 (Rails 8.1) the app crashes on boot whenever this plugin is installed. In production Rails eager-loads all controllers, OauthProvidersController is loaded, and it raises:

application_controller.rb:39:in 'model_object': wrong number of arguments (given 1, expected 0) (ArgumentError)
	from app/controllers/oauth_providers_controller.rb:24:in '<class:OauthProvidersController>'

Redmine 7.0 removed the class-level model_object DSL helper from ApplicationController (trunk r24017 / redmine.org #43205, #43230) and migrated its own controllers to assign the class_attribute directly (e.g. self.model_object = Document). model_object is now only the 0-arg reader, so model_object OauthProvider raises.

Fix

- model_object OauthProvider
+ self.model_object = OauthProvider

class_attribute :model_object generates the model_object= writer in both Redmine 6.x and 7.x, so this keeps working under the current requires_redmine 6.0.0.

Testing

Redmine 7.0.0 (Rails 8.1.3), Ruby 3.4.8:

• Before: the app exits on boot with the ArgumentError above and never becomes healthy (restart loop).
• After: the app boots normally, and the full Cloudron app lifecycle test passes (51/51), including the OAuth/OIDC login flow that routes through OauthProvidersController, plus backup/restore, in-place update, and location change.

[picman]

Thank you very much!