Add DI tests for LightInject, StashBox, Lamar

[jithu7432]

As discussed on #1134, this is an attempt to add the remaining providers.

[Copilot]

Pull Request Overview

This PR adds dependency injection test coverage for three additional IoC containers: LightInject, StashBox, and Lamar. The changes extend the existing test infrastructure to verify that MediatR works correctly with these DI providers.

Key changes:

• Added test fixtures for LightInject, StashBox, and Lamar DI containers
• Created corresponding test classes that inherit from BaseAssemblyResolutionTests
• Updated project dependencies to include the necessary adapter packages

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
StashBoxDependencyInjectionTests.cs	Test class for StashBox container integration
StashBoxServiceProviderFixture.cs	Service provider fixture implementing StashBox container setup
LightInjectServiceProviderFixture.cs	Service provider fixture implementing LightInject container setup
LamarServiceProviderFixture.cs	Service provider fixture implementing Lamar container setup
MediatR.DependencyInjectionTests.csproj	Added package references for the three new DI containers
LightInjectDependencyInjectionTests.cs	Test class for LightInject container integration
LamarDependencyInjectionTests.cs	Test class for Lamar container integration

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