Add v_total_direct_service_children virtual total to Service model

[GilbertCherrie]

This PR is required for: ManageIQ/manageiq-ui-service#2215 and ManageIQ/manageiq-api#1345

Add v_total_direct_service_children virtual total to Service

Adds a new SQL-backed virtual total v_total_direct_service_children to the Service model that counts the number of direct (first-level) service children for a given service record.

What it does:

Uses an Arel subquery against the services table to count child records whose ancestry exactly matches the current record's ancestry path
Handles both root services (no ancestry) and nested services (with ancestry) via a CASE expression
Returns 0 (via COALESCE) when no direct children exist
Supports SQL-level evaluation (via SELECT) and falls back to Ruby-level evaluation

Tests added:

SQL path: verifies v_total_direct_service_children is populated correctly when selected via Service.select(:id, :v_total_direct_service_children)
Ruby path: verifies the attribute is computed in Ruby when not SQL-selected, and confirms attribute_present? returns false in that case

[GilbertCherrie]

@miq-bot assign @jrafanie
@miq-bot add-label enhancement

[Fryguy]

"direct" in our parlance means "directly under the service", so in the example tree below there are only 2 direct service children, c1, and c2. Are you looking for all service children (i.e. the entire tree?)

[Fryguy]

Ok yeah - spoke with @GilbertCherrie and that is what this is doing - the test didn't make that clear to me.

[GilbertCherrie]

@miq-bot add-label tal/yes?

[Fryguy]

I'm going to merge this as is, however I want to follow up with the ancestry gem and see if we can find a way to expose it there instead, because it's really a concern for that gem. Spoke to @kbrock about it briefly, and he agreed, though we're not actually sure how to structure that, hence merging this as is.

[miq-bot]

Checked commits GilbertCherrie/manageiq@b34a429~...8b48ed8 with ruby 3.3.12, rubocop 1.91.0, haml-lint 0.78.0, and yamllint 1.37.1
2 files checked, 0 offenses detected
Everything looks fine. 🍪

[Fryguy]

Backported to tal in commit c519f17.

commit c519f17296a5ff26e43e6fb6c25579dfece15f6b
Author: Jason Frey <fryguy9@gmail.com>
Date:   Fri Oct 2 18:26:37 2026 -0400

    Merge pull request #23995 from GilbertCherrie/add-v-total-direct-service-children-virtual-total
    
    Add v_total_direct_service_children virtual total to Service model
    
    (cherry picked from commit 27efeccb539d22e625455f4bd7df4853a153cc7b)