Category product count does not include products assigned to the category itself

[mimou78]

Summary

This pull request fixes an issue where the product count displayed in the Admin Category Grid
incorrectly shows 0 products for categories that do not have any descendants.
The current logic only counts products assigned to descendant categories and does not
include products directly assigned to the category itself.

Problem

When building the temporary table of category → descendant relationships, the SQL
logic only inserts rows for descendants, not for the category itself.
As a result:

• Leaf categories always show 0 products
• Parent categories show incomplete counts
• The Admin UI becomes misleading for merchants

Related Issue
Fixes #40263

What This Fix Does

This change ensures that each category always includes a self-reference entry in
the temporary category/descendant table.
As a result:

• Products directly assigned to the category are counted correctly
• Categories without children no longer show a zero count
• Product assignment visibility in Admin becomes accurate

Technical Changes

• Added a self-reference row (category_id = X, descendant_id = X) for every
  processed category ID.
• Ensured that the final SQL aggregation counts products from both descendants
  and the category itself.
• Updated unit tests to validate multiple insert calls for self-references.

Testing Instructions

1. Create a category with no children.
2. Assign one or more products to it.
3. Clear cache and refresh the Category Grid.
4. Product count should now display the correct value instead of zero.
5. Repeat with nested categories to confirm descendant counts still work.

Backward Compatibility

• No schema changes.
• No API changes.
• Fully backward compatible.

Risks

Low. This affects only the temporary table generation used for counting, not
the category model or product relations.

[m2-assistant]

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[ct-prd-pr-scan]

The security team has been informed about this pull request due to the presence of risky security keywords. For security vulnerability reports, please visit Adobe's vulnerability disclosure program on HackerOne or email psirt@adobe.com.

[ct-prd-pr-scan]

The security team has been informed about this pull request due to the presence of risky security keywords. For security vulnerability reports, please visit Adobe's vulnerability disclosure program on HackerOne or email psirt@adobe.com.

[mimou78]

@magento run all tests