Implement customer overdue orders query feature

[Copilot]

Adds ability to list customers with overdue orders (DueDate < Today AND Status ∉ {Shipped, Cancelled}), ordered by oldest overdue date ascending.

Changes

• Contract: ICustomerService.GetCustomersWithOverdueOrders() returns CustomerWithOverdueOrdersData[] with customer name, overdue count, and oldest due date
• Service: Single LINQ query performs filtering, aggregation (Count(), Min()), and sorting at database level
• Console: New menu option "Show customers with overdue orders"
• Tests: 12 unit tests covering edge cases, status/date filtering, aggregation, and sorting logic using FakeRepository

Implementation

public CustomerWithOverdueOrdersData[] GetCustomersWithOverdueOrders()
{
    var today = DateTime.Today;
    var closedStatuses = new[] { SalesOrderHeaderStatusValues.Shipped, SalesOrderHeaderStatusValues.Cancelled };

    return repository.GetEntities<Customer>()
        .Where(c => c.SalesOrderHeaders.Any(o => 
            o.DueDate < today && 
            !closedStatuses.Contains(o.Status)))
        .Select(c => new CustomerWithOverdueOrdersData
        {
            CustomerName = c.CompanyName ?? $"{c.FirstName} {c.LastName}",
            OverdueOrdersCount = c.SalesOrderHeaders
                .Where(o => o.DueDate < today && !closedStatuses.Contains(o.Status))
                .Count(),
            OldestDueDate = c.SalesOrderHeaders
                .Where(o => o.DueDate < today && !closedStatuses.Contains(o.Status))
                .Min(o => o.DueDate)
        })
        .OrderBy(x => x.OldestDueDate)
        .ToArray();
}

Customer name resolution: CompanyName if present, otherwise FirstName + LastName.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Implement: Show customer with overdue orders</issue_title>
<issue_description>Implement feature #1 using high-level design from docs/workitems/1-design.md and detailed design from docs/workitems/1-detailed-design.md</issue_description>

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

Custom agent used: coder
Implementation agent that converts detailed design specifications into working C# code following Clean Architecture principles and strict dependency rules

• Fixes Implement: Show customer with overdue orders #9

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