[5.3] Show correct custom fields when creating article after filtering by multiple categories

[AdarshSantoria]

Pull Request for Issue #45303

Summary of Changes

This PR fixes a bug where creating a new article after filtering by multiple categories (e.g. "News" and "Events") incorrectly displays all custom fields from all those categories on the "Fields" tab.

Joomla previously used the catid filter directly from user state or input, which led to ambiguous behavior when multiple categories were selected. This fix:

• Detects when category_id filter is an array
• Selects the first selected category from the filtered list
• Ensures only relevant custom fields are loaded for the selected category
• Supports fallback logic for single-category filter or default category

This resolves #45303 and improves clarity and correctness of the custom field rendering in the admin UI.

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Testing Instructions

1. Create two categories: Events and News
2. Create two custom fields:
   • events_title → assigned to Events
   • news_title → assigned to News
3. Go to Content > Articles
4. Apply category filter for both "Events" and "News"
5. Click New Article
   • ✅ Only one custom field (events_title or news_title) should appear on the "Fields" tab
6. Change category using dropdown
   • ✅ Custom fields update correctly

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Actual result BEFORE applying this Pull Request

• When filtered by multiple categories and clicking "New", all fields from all selected categories are loaded.
• Changing category does not reload the field list properly.

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Expected result AFTER applying this Pull Request

• Only the first selected category’s custom fields are loaded initially
• Changing category in the form correctly reloads the associated custom fields dynamically
• Prevents confusion and clutter in the custom fields UI

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Link to documentation

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[brianteeman]

I have tested this item ✅ successfully on 56c2e62

Functionality works as described

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45353.}

[JackJoeJack]

I also have tested this item ✅ successfully on 56c2e62

Functionality works as described

[QuyTon]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/45353.}

[richard67]

I've just double-checked this PR and it works and seems not to break anything else.

[richard67]

Thanks all (author and testers).