Add MassTransit and NServiceBus shim interfaces for migration

[jeremydmiller]

Summary

• Add shim interfaces in Wolverine.Shims.MassTransit (IConsumer<T>, ConsumeContext<T>, IPublishEndpoint, ISendEndpointProvider) and Wolverine.Shims.NServiceBus (IHandleMessages<T>, IMessageHandlerContext, IMessageSession, IEndpointInstance, IUniformSession, ITransactionalSession) that delegate to Wolverine's IMessageBus/IMessageContext
• All shim handler interfaces extend IWolverineHandler for automatic handler discovery — no explicit registration calls needed for IConsumer<T> or IHandleMessages<T> handlers
• Code generation variable sources are auto-registered in WolverineOptions so ConsumeContext<T> and IMessageHandlerContext are resolved inline without service location
• MessageBus directly implements IPublishEndpoint and ISendEndpointProvider via a partial class
• Added [WolverineMessageWrapper] attribute and updated MethodInfoExtensions.MessageType() to unwrap generic wrapper types (e.g. ConsumeContext<T> → T) for correct message type discovery
• Updated migration documentation with auto-discovery notes and handler discovery links

Closes #2202

Test plan

• 72 shim tests pass (unit tests for all delegation + end-to-end integration tests for both MassTransit and NServiceBus handlers with cascading messages)
• All 1122 CoreTests pass
• Verify docs render correctly in VitePress

🤖 Generated with Claude Code