Migrate to .NET 9 with Microsoft.Extensions.DependencyInjection

[erichexter]

Summary

• Drop all 6 legacy IoC container integrations (Autofac, Ninject, SimpleInjector, StructureMap, Unity, Windsor) — all are unmaintained, deprecated, or superseded by the standard .NET DI system
• Add ShortBus.Extensions.DependencyInjection — a new integration package for Microsoft.Extensions.DependencyInjection, the standard DI container since .NET Core 1.0
• Migrate all projects to SDK-style .csproj, targeting netstandard2.0 + net9.0
• Add GitHub Actions CI with NuGet publish on v* tags
• Update README with modern setup examples

New setup (3 lines)

services.AddShortBus();
services.AddShortBusHandlers(typeof(Program).Assembly);
var mediator = provider.GetRequiredService<IMediator>();

Breaking changes

• Requires .NET Standard 2.0 or .NET 9 (drops .NET Framework 4.5 direct support, though netstandard2.0 covers .NET Framework 4.6.1+)
• The 6 IoC container packages are removed — projects using them will need to switch to ShortBus.Extensions.DependencyInjection or implement IDependencyResolver directly
• Derived-type handler resolution (e.g. registering a handler for Ping and calling with PingALing : Ping) was previously supported implicitly by StructureMap's variance handling but is not supported by MEDI's exact-type matching

Tests

8 tests passing covering: request/response, async request/response, multi-interface handlers, notifications, and container resolution.

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