[rc2] Fix MissingMethodException when abstract IDesignTimeServices classes are discovered

[Copilot]

Fixes #34617

Description
The EF Core design-time services discovery mechanism was attempting to instantiate abstract classes implementing IDesignTimeServices, which resulted in a MissingMethodException when running commands like dotnet ef migrations add.

Customer impact
All types implemented IDesignTimeServices have to be made instantiable as a work-around.

How found
Customer reported on 8.0.8

Regression
No

Testing
Tests added

Risk
Low. The fix is very localized.

Example scenario that failed before this fix:

[assembly: DesignTimeServicesReference("Data.Entity.ActualDesignTimeServices, MyProject")]

public abstract class DesignTimeServicesBase : IDesignTimeServices
{
    public void ConfigureDesignTimeServices(IServiceCollection serviceCollection)
    {
        DoSomething();
    }

    protected abstract void DoSomething();
}

public class ActualDesignTimeServices : DesignTimeServicesBase
{
    protected override void DoSomething()
    {
        // Implementation
    }
}

Running dotnet ef migrations add would throw:

System.MissingMethodException: Cannot dynamically create an instance of type 'DesignTimeServicesBase'. 
Reason: Cannot create an abstract class.

Fix

Modified the type discovery filter to use IsInstantiable() instead of checking only !IsAbstract:

.Where(t => typeof(IDesignTimeServices).IsAssignableFrom(t) && t.IsInstantiable())

The IsInstantiable() extension method provides comprehensive filtering by excluding:

• Abstract classes (which cannot be instantiated)
• Interfaces (which cannot be instantiated)
• Generic type definitions (which cannot be instantiated)

This change:

• Aligns with EF Core's existing pattern for finding constructible types in GetConstructibleTypes()
• Only affects the automatic discovery mechanism - explicit type references via DesignTimeServicesReferenceAttribute are unchanged
• Provides more robust type filtering than just checking !IsAbstract

Testing

Added comprehensive tests covering:

• Abstract base classes with concrete implementations
• Interfaces implementing IDesignTimeServices
• Mixed scenarios with both abstract and concrete types
• Verification that concrete types are properly selected

All existing tests continue to pass, confirming no regressions in the design-time services functionality.