Pull Request resolves #48207

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

This PR refactors com_tags to use the same parent-scoped alias architecture as com_content and com_categories. The alias uniqueness check in TagTable::store() is now scoped to parent_id so that aliases are unique among siblings but can be reused under different parents. Additionally, TagModel::save() has been updated with the standard "empty alias handling" logic to auto generate aliases from the title and automatically increment them if a sibling duplicate exists, triggering the standard UI warning banner instead of a hard validation error.

Testing Instructions

1. Create a new Tag titled "Parent 1".
2. Create a new Tag titled "Parent 2".
3. Create a new Tag titled "Child" with "Parent 1" as the parent. Leave the alias blank. Save & Close.
4. Create a second new Tag titled "Child" with "Parent 1" as the parent. Leave the alias blank. Save & Close.
5. Create a third new Tag titled "Child" with "Parent 2" as the parent. Leave the alias blank. Save & Close.

Actual result BEFORE applying this Pull Request

Step 3: Succeeds, alias is child.
Step 4: Fails with error: "Save failed with the following error: Another Tag has the same alias."
Step 5: Fails with error: "Save failed with the following error: Another Tag has the same alias." (Globally scoped error, despite different parent).

Expected result AFTER applying this Pull Request

Step 3: Succeeds, alias is child.
Step 4: Succeeds, but shows a warning: "Alias already existed so a number was added at the end. You can re-edit the tag to customise the alias." The saved alias is auto-incremented to child-2.
Step 5: Succeeds without warnings, alias is child. (Because it is under a different parent).

Link to documentations

Please select:

• Documentation link for guide.joomla.org:

• No documentation changes for guide.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed