Don't register message types as services in lazy caching mode

[yvanlo]

Fixes #278

Problem

In CachingMode.Lazy, handler wrappers were registered in DI using the message type as the service type:

services.Add(new ServiceDescriptor(typeof(MyCommand), _ => new CommandHandlerWrapper<MyCommand, string>(), Singleton));

ASP.NET Core Minimal APIs use IServiceProviderIsService to decide where to bind a parameter from. MyCommand was reported as a service, so it was bound from DI instead of the request body. The result was an InvalidCastException.

Fix

• Wrappers are registered by their own type (RequestHandlerWrapper<,>, NotificationHandlerWrapper<>, ...), and the typed Send/Publish/CreateStream methods resolve them by that type.
• For lookups based on the runtime type (projects above the "many messages" threshold), ContainerMetadata now has a HandlerWrapperTypes map from message type to wrapper type. It only holds typeof pairs, so the wrappers themselves are still resolved lazily.
• Eager mode is unchanged.

Commits

1. The template fix
2. A test that message types are not registered as services (IServiceProviderIsService + GetService)
3. Updated lazy snapshots (only cm=L snapshots change)

Testing

• The new test fails on main in Lazy mode (7/7) and passes with the fix
• Mediator.Tests on net10.0: all lifetimes (Singleton/Scoped/Transient) × sizes (Default/Large) × caching modes (Eager/Lazy) pass
• Mediator.SourceGenerator.Tests: 182/190 pass. The 8 SampleTests failures are the same as on main (NuGet audit warnings), fixed by Bump Scriban to 7.4.0 to fix known vulnerabilities #288 and Bump sample package versions to fix known vulnerabilities #289