[Mono.Android] Wrap connection exceptions in HttpRequestException (do…

…tnet#7661)

Fixes: dotnet#7629

Whenever a Java backend connection fails for any reason, wrap the
thrown exception in `HttpRequestException` as described in the
[`HttpClient.SendAsync()` documentation][0].

[0]: https://learn.microsoft.com/en-us/dotnet/api/system.net.http.httpclient.sendasync?view=net-7.0#system-net-http-httpclient-sendasync(system-net-http-httprequestmessage)

src/Mono.Android/Xamarin.Android.Net/AndroidMessageHandler.cs

@@ -494,7 +494,11 @@ Task ConnectAsync (HttpURLConnection httpConnection, CancellationToken ct)
 						Logger.Log (LogLevel.Info, LOG_APP, $"Exception caught while cancelling connection: {ex}");
 						ct.ThrowIfCancellationRequested ();
 					}
-					throw;
+
+					// All exceptions related to connectivity should be wrapped in HttpRequestException. In theory it is possible that the exception will be
+					// thrown for another reason above, but it's OK to wrap it in HttpRequestException anyway, since we're working in the context of
+					// `ConnectAsync` which, from the end user's point of view, is 100% related to connectivity.
+					throw new HttpRequestException ("Connection failure", ex);
 				}
 			}, ct);
 		}

tests/Mono.Android-Tests/Xamarin.Android.Net/AndroidMessageHandlerTests.cs

@@ -139,10 +139,14 @@ private async Task AssertRejectsRemoteCertificate (Func<Task> makeRequest)
 				Assert.Fail ("The request wasn't rejected");
 			}
 #if NET
-			catch (System.Net.WebException ex) {}
+			// While technically we should be throwing only HttpRequestException (as per HttpClient.SendAsync docs), in reality
+			// we need to consider legacy code that migrated to .NET and may still expect WebException.  Thus, we throw both
+			// of these and we need to catch both here
+			catch (System.Net.WebException) {}
 #else
 			catch (Java.IO.IOException) {}
 #endif
+			catch (System.Net.Http.HttpRequestException) {}
 		}
 	}
 }