[dotnet watch] Enable CA2007 (ConfigureAwait) for startup hook assembly

src/Dotnet.Watch/HotReloadAgent.Host/.editorconfig

@@ -3,4 +3,8 @@
 # IDE0240: Remove redundant nullable directive
 # The directive needs to be included since all sources in a source package are considered generated code
 # when referenced from a project via package reference.
-dotnet_diagnostic.IDE0240.severity = none
+dotnet_diagnostic.IDE0240.severity = none
+
+# CA2007: This assembly runs inside the user's app process where a SynchronizationContext may be present.
+# Missing ConfigureAwait(false) can cause deadlocks (e.g. iOS UIKitSynchronizationContext).
+dotnet_diagnostic.CA2007.severity = error

src/Dotnet.Watch/HotReloadAgent.Host/NamedPipeTransport.cs

@@ -34,8 +34,8 @@ public override async ValueTask SendAsync(IResponse response, CancellationToken
             }
         }
 
-        await _pipeClient.WriteAsync((byte)response.Type, cancellationToken);
-        await response.WriteAsync(_pipeClient, cancellationToken);
+        await _pipeClient.WriteAsync((byte)response.Type, cancellationToken).ConfigureAwait(false);
+        await response.WriteAsync(_pipeClient, cancellationToken).ConfigureAwait(false);
     }
 
     public override ValueTask<RequestStream> ReceiveAsync(CancellationToken cancellationToken)

src/Dotnet.Watch/HotReloadAgent.Host/StartupHook.cs

@@ -84,7 +84,7 @@ async Task SendAndForgetAsync()
                 {
                     try
                     {
-                        await listener.SendResponseAsync(new HotReloadExceptionCreatedNotification(code, message), CancellationToken.None);
+                        await listener.SendResponseAsync(new HotReloadExceptionCreatedNotification(code, message), CancellationToken.None).ConfigureAwait(false);
                     }
                     catch
                     {