Transition plugin (#10088)

* Add new enum

* Define StateTransitionMixin class

* Import

* Rename plugin mixin class

- Avoid naming clash

* Offload transitions to plugin registry

* fix imports

* Fix default value

* Fix sample transition class

* Refactor unit test

* Add docs

* Fix type hinting

* Check for expected message

* Tests for various failure modes

* Tweak query count limit

* Exclude lines from coverage

* Remove debug code

---------

Co-authored-by: Matthias Mair <code@mjmair.com>

docs/docs/plugins/mixins/transition.md

@@ -0,0 +1,16 @@
+---
+title: Transition Mixin
+---
+
+## TransitionMixin
+
+The `TransitionMixin` allows plugins to provide custom state transition logic for InvenTree models.
+
+Model types which support transition between different "states" (e.g. orders), can be extended to support custom transition logic by implementing the `TransitionMixin` class.
+
+This allows for custom functionality to be executed when a state transition occurs, such as sending notifications, updating related models, or performing other actions.
+
+Additionally, the mixin can be used to prevent certain transitions from occurring, or to modify the transition logic based on custom conditions.
+
+!!! info "More Info"
+    For more information on this plugin mixin, refer to the InvenTree source code. [A working example is available as a starting point]({{ sourcefile("/src/backend/InvenTree/plugin/samples/integration/transition.py") }}).

docs/mkdocs.yml

@@ -233,6 +233,7 @@ nav:
       - Report Mixin: plugins/mixins/report.md
       - Schedule Mixin: plugins/mixins/schedule.md
       - Settings Mixin: plugins/mixins/settings.md
+      - Transition Mixin: plugins/mixins/transition.md
       - URL Mixin: plugins/mixins/urls.md
       - User Interface Mixin: plugins/mixins/ui.md
       - Validation Mixin: plugins/mixins/validation.md