fix: translation_key class attribute

[Paul-Bob]

Description

Fixes #3538

The translation_key method defined in the base resource relied on an instance attribute, @translation_key, which was always nil.

    class << self
        def translation_key
          @translation_key || "avo.resource_translations.#{class_name.underscore}"
        end
    end

Previously, a bug in Rails (prior to this Rails PR) inadvertently redefined the class attribute reader for all sub-classes that defined the class attribute, causing translation_key to exhibit the expected behavior by chance. After the fix in Rails 8.0.1, the correct reader from the parent class is now used. However, since @translation_key is an instance attribute rather than a class-level attribute, it remains nil, and the method defaults to the fallback value. Check rails/rails#53992 for more details.

To fix this, I renamed the class attribute to custom_translation_key and aliased the writer to translation_key for backward compatibility.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

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This PR has been merged into main. The functionality will be available in the next release.

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