Add a new UseKestel API to the WebApplicationFactory

[mkArtakMSFT]

• Added UseKestrel() API to the WebApplicationFactory type. The API configures the WAF, so that later during initialization it will use Kestrel, instead of a TestServer for WebHostBuilder-based applications.
• Renamed the EnsureServer() private method to Initialize() and made it public, so that consumders can call it directly, without the need of creating a client, as in many situations customers didn't need a client to interact with.

This is an alternative design to enabling real server usage with WebApplicationFactory, which was considered here: #60247

[KSemenenko]

This is an amazing idea!

[Copilot]

PR Overview

This PR introduces an alternative design for enabling real server usage with WebApplicationFactory by adding a new API that leverages Kestrel. Key changes include:

• Adding a new test class (RealServerBackedIntegrationTests.cs) that validates the real server integration.
• Introducing a new WebApplicationFactory subclass (KestrelBasedWapFactory.cs) that automatically enables Kestrel.
• Updating the core WebApplicationFactory implementation to conditionally use a Kestrel server versus the traditional TestServer.

Reviewed Changes

File	Description
src/Mvc/test/Mvc.FunctionalTests/RealServerBackedIntegrationTests.cs	New integration tests for real server backed functionality.
src/Mvc/test/Mvc.FunctionalTests/KestrelBasedWapFactory.cs	A factory that configures the WebApplicationFactory to use Kestrel.
src/Mvc/Mvc.Testing/src/WebApplicationFactory.cs	Modifications to support the new UseKestrel API, including updated server creation, client configuration, and error handling.

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.