Add baseline unit coverage and Verify-based public API approvals; wire CI to solution tests

[Copilot]

This PR adds initial automated coverage for the testing factories and introduces approval testing for the package public surface using PublicApiGenerator + Verify. CI is aligned to build/test the solution explicitly so the new test project is always exercised.

• Test coverage foundation

  • Added ServiceComposer.AspNetCore.Testing.Tests (xUnit) and included it in the solution.
  • Added focused tests for:
    • SelfContainedWebApplicationFactoryWithHost<TEntryPoint>
    • SelfContainedWebApplicationFactoryWithWebHost<TEntryPoint>
    • WebApplicationFactoryWithWebHost<TStartup>
  • Coverage targets request handling and builder customization hooks.

• Public API approval testing

  • Replaced custom reflection/string-comparison API snapshot logic.
  • Added package-based approval flow:
    • PublicApiGenerator for API extraction
    • Verify.Xunit for snapshot verification
  • Switched from custom PublicApi.approved.txt to Verify .verified.txt baseline artifact.

• CI execution scope

  • Updated workflow commands to target src/ServiceComposer.AspNetCore.Testing.sln for both build and test.
  • Ensures test project participation is deterministic in CI.

[Fact]
public Task Public_api_is_approved()
{
    var publicApi = ApiGenerator.GeneratePublicApi(typeof(WebApplicationFactoryWithWebHost<>).Assembly);
    return Verifier.Verify(publicApi);
}

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