Fix/1877 notification shapes

[MichaelL0827]

Summary

Closes #1877

Normalizes notification values returned from before-action and after-action hooks.

Hooks can now return notifications as:

• A single notification
• A list of notifications
• A map containing a :notifications key
• nil

This behavior is shared across Ash.ActionInput and Ash.Changeset hooks.

Note: the linked issue only mentioned Ash.Changeset, but Ash.ActionInput had the identical inconsistency (same two shapes, same crash pattern), so it's fixed here as well for consistency.

Root Cause

Ash.Notifier.Notification had no shared type or normalization function for its hook-return shapes. As a result, action_input.ex and changeset.ex each accepted a different shape at different call sites: before_action hooks expected %{notifications: [...]}} while after_action hooks expected a bare list. Returning the "wrong" shape for a given hook crashed with a BadMapError or KeyError rather than working consistently.

Changes

• Notification.ex: added Ash.Notifier.Notification.input/0 (the union type) and normalize/1, which converts any accepted shape into a flat list.
• Changeset.ex: run_before_actions and run_after_actions now call normalize/1 instead of pattern-matching/List.wrap-ing a single hardcoded shape.
• action_input.ex: identical changes to run_before_actions and merge_after_action_notifications.
• Updated docs in action_input.ex, changeset.ex, and actions.md to reflect the accepted notification shapes.
• Added regression tests for notification return shapes in notifier_test.exs and generic_actions_test.exs.

Testing

• added tests: "a custom notification can be returned as a bare list from an after_action hook", "a custom notification can be returned as a map from an after_action hook", "notifications from before and after action hooks are both delivered", "all notifications returned from an after_action hook are delivered", "a custom notification can be returned as a bare list from a before_action hook", "before_action hooks can return notifications as a bare list", "after_action hooks can return notifications as a map"
• Tests fail without this change: yes
• mix check and mix dialyzer pass locally: yes

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• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

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