Use WebApplication in unit tests

[erwinkramer]

This pull request refactors the test server setup in several test classes to use the newer WebApplication and minimal API patterns introduced in ASP.NET Core, replacing the older HostBuilder and IApplicationBuilder approach. This change simplifies server configuration, makes the test code more concise, and aligns it with modern ASP.NET Core best practices.

Test server setup modernization:

• Replaced usage of HostBuilder and IApplicationBuilder with WebApplication.CreateBuilder() and minimal API endpoint mapping (e.g., app.MapGet, app.MapPost) in TestApiServer, TestOAuthServer, and TestClientCredentialsServer, as well as in integration test helpers. This results in less boilerplate and easier-to-read test server setup. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Test helper refactoring:

• Updated test helper methods such as CreateHostAsync in integration tests to use WebApplication and minimal APIs, improving consistency and reducing code duplication. [1] [2] [3]

Type and method signature updates:

• Changed signatures of private configuration methods to accept WebApplication instead of IApplicationBuilder to match the new server setup approach. [1] [2]

These changes modernize the test infrastructure, making it easier to maintain and extend in the future.

[abdebek]

Awesome! Thanks!