fix: add catch-all can?/3 clause for custom item actions

[pshoukry]

Summary

• Fixes crash when using custom item_actions in AshBackpex.LiveResource
• Adds a fallback can?/3 clause that handles custom actions

Problem

When using custom item_actions in an AshBackpex.LiveResource, the page crashes with:

no function clause matching in MyAppWeb.Admin.UserLive."-inlined-can?/3-"/3

This happens because the transformer generates specific can?/3 function clauses for standard actions (:new, :index, :show, :edit, :delete) but does NOT generate a catch-all fallback clause.

Solution

Added a catch-all can?/3 clause after the destroy_action block that:

1. Returns true for actions that don't exist on the Ash resource (custom UI actions)
2. Checks Ash authorization using Ash.can? for actions that do exist on the resource

This mirrors the base Backpex.LiveResource behavior (which returns true by default) while integrating with Ash's authorization system when applicable.

Test plan

• Added TestPromoteItemAction - a custom item action with icon for testing
• Added TestCustomItemActionLive - a LiveResource with the custom item action
• Added tests verifying:
  • Unknown actions (like :promote) return true
  • Known Ash actions (like :read) check Ash authorization
• All existing tests pass

[enoonan]

Awesome work. Thank you for this!

[pshoukry]

🙇 Thanks for merging!

[enoonan]

Just pushed the new version to Hex and created a release