Implement AsSplitQuery support in AsSplitQueryEvaluator

[Copilot]

AsSplitQueryEvaluator had the core AsSplitQuery() call permanently commented out with a TODO citing lack of EF Core support. The project now targets EF Core 9.x (AsSplitQuery available since EF Core 5.0), so this evaluator was silently a no-op despite ISpecification<T>.AsSplitQuery being set.

Changes

• AsSplitQueryEvaluator.cs: Uncomment and activate query = query.AsSplitQuery(), add missing using Microsoft.EntityFrameworkCore import, remove stale TODO comment

// Before: silently ignored
if (specification.AsSplitQuery)
{
    //query = query.AsSplitQuery(); // TODO: Support for split query is not in the current EF Core version.
}

// After: consistent with AsNoTrackingEvaluator pattern
if (specification.AsSplitQuery)
{
    query = query.AsSplitQuery();
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix code scanning alert - A "TODO" or similar was left in source code, possibly indicating incomplete functionality</issue_title>
<issue_description>

Tracking issue for:

• https://github.com/improvgroup/sharedcode/security/code-scanning/8
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix code scanning alert - A "TODO" or similar was left in source code, possibly indicating incomplete functionality #90

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