fix(tests): replace obsolete SelectAwait with Select in HttpClientTests

Remove conditional compilation and use Select with CancellationToken
parameter for all target frameworks (net8.0, net9.0, net10.0) to resolve
CS0618 warning about obsolete AsyncEnumerableDeprecated.SelectAwait.

The modern Select overload is available in both System.Linq.Async
(for net8.0/net9.0) and System.Linq.AsyncEnumerable (for net10.0).

Author: devlead

src/Devlead.Testing.MockHttp.Tests/Unit/HttpClientTests.cs

@@ -92,13 +92,8 @@ public async Task Send(params string[] methods)
             // When
             var result = await methods
                             .ToAsyncEnumerable()
-#if NET10_0_OR_GREATER
                             .Select(
                                 async (method, _, _)
-#else
-                            .SelectAwait(
-                                async method 
-#endif
                                     => await httpClient.SendAsync(
                                                     new HttpRequestMessage(
                                                         new HttpMethod(method),