Fixes 18555: Fix model URL generator for plugins

[alehaa]

Fixes: #18555

The default get_absolute_url() implementation can now return the URL for plugins.

[jeremystretch]

No issues with the fix itself, but I'd like to see if we can find a less disruptive solution for testing.

[jeremystretch]

This change has implications beyond the scope of the bug report. Most importantly, we specifically want to test support for basic Django models introduced in plugins.

Rather than modify the core test model, it might be possible to use Django's isolate_apps utility to create a NetBoxModel subclass temporarily, specifically to test the functionality of its get_absolute_url() method. Something like this could work:

class ModelTest(TestCase):

    @isolate_apps('dummy_plugin')
    def test_get_absolute_url(self):
        class TestModel(NetBoxModel):
            pass

        model = TestModel()
        model.pk = 123
        self.assertEqual(TestModel.get_absolute_url(), f'/plugins/dummy-plugin/models/{m.pk}/')