Add ExtensionLoader with DI support for assembly-scanned plugin extensions

[Copilot]

TypeLoader<T> and ExtensionInstantiator existed independently with no unified entry point that tied assembly scanning, DI-aware instantiation, and service-container registration together. This adds ExtensionLoader<TExtension> to close that gap.

New types

• IExtensionLoader<TExtension> — interface exposing LoadFromAssembly, LoadFromAssemblies, CreateInstance(Guid), CreateAllInstances(), and RegisterExtensions(IServiceCollection, ServiceLifetime)
• ExtensionLoader<TExtension> — concrete implementation; wraps TypeLoader<TExtension> for discovery and ActivatorUtilities.CreateInstance so extension constructors receive registered services automatically

DI registration

Added AddExtensionLoader<TExtension>() to ServiceCollectionExtensions, registered as singleton:

services.AddExtensionLoader<IMyExtension>();

// Later, or at startup:
var loader = provider.GetRequiredService<IExtensionLoader<IMyExtension>>();
loader.LoadFromAssembly(path);

// Create instances with full DI resolution
var instances = loader.CreateAllInstances();

// Or register discovered types back into a service collection
loader.RegisterExtensions(services, ServiceLifetime.Transient);

RegisterExtensions registers each discovered type under both its concrete type and TExtension, enabling IEnumerable<TExtension> resolution from the container.

Tests

Added 8 tests to ExtensionLoadingIntegrationTests covering: constructor null-guard, empty state, CreateInstance with unknown GUID, CreateAllInstances with no types, RegisterExtensions null-guard and no-op paths, singleton lifetime via AddExtensionLoader, and interface implementation verification.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Extension Loading with DI</issue_title>
<issue_description>Update ExtensionLoader to use IServiceProvider and register extensions in service container.

Acceptance Criteria:

• ExtensionLoader receives IServiceProvider
• Extensions registered with proper lifetimes
• Extension dependencies resolved through DI
• Extension initialization updated
• Tests for extension loading
• Documentation updated

Technical Details:

• Pass IServiceProvider to extension constructors
• Register extension types in container
• Support for extension dependencies
• Proper disposal of scoped services

Files to Modify:

• Libraries/ACATCore/Utility/TypeLoader/ExtensionLoader.cs
• Extension base classes
• Extension registration code

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Epic: Dependency Injection Infrastructure

Estimated Effort: 3 days

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Parent Feature: #190 - Dependency Injection Infrastructure

</issue_description>

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

• Fixes Extension Loading with DI #216

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