Fixes GroupBy does not invoke decider

src/core/Akka.API.Tests/CoreAPISpec.ApproveStreams.verified.txt

@@ -1012,6 +1012,10 @@ namespace Akka.Streams
         Shaping = 0,
         Enforcing = 1,
     }
+    public class TooManySubstreamsOpenException : System.InvalidOperationException
+    {
+        public TooManySubstreamsOpenException() { }
+    }
     public abstract class TransformerLikeBase<TIn, TOut> : Akka.Streams.ITransformerLike<TIn, TOut>
     {
         protected TransformerLikeBase() { }

src/core/Akka.Streams.Tests/Dsl/FlowGroupBySpec.cs

@@ -427,6 +427,20 @@ public void GroupBy_must_fail_when_exceeding_maxSubstreams()
             }, Materializer);
         }
 
+        [Fact]
+        public void GroupBy_must_resume_when_exceeding_maxSubstreams()
+        {
+            var f = Flow.Create<int>().GroupBy(0, x => x).MergeSubstreams();
+            var (up, down) = ((Flow<int, int, NotUsed>)f)
+                .WithAttributes(ActorAttributes.CreateSupervisionStrategy(Deciders.ResumingDecider))
+                .RunWith(this.SourceProbe<int>(), this.SinkProbe<int>(), Materializer);
+
+            down.Request(1);
+
+            up.SendNext(1);
+            down.ExpectNoMsg(TimeSpan.FromSeconds(1));
+        }
+
         [Fact]
         public void GroupBy_must_emit_subscribe_before_completed()
         {

src/core/Akka.Streams/Implementation/Fusing/StreamOfStreams.cs

@@ -411,7 +411,7 @@ public void OnPush()
                     else
                     {
                         if (_activeSubstreams.Count == _stage._maxSubstreams)
-                            Fail(new IllegalStateException($"Cannot open substream for key {key}: too many substreams open"));
+                            throw new TooManySubstreamsOpenException();
                         else if (_closedSubstreams.Contains(key) && !HasBeenPulled(_stage.In))
                             Pull(_stage.In);
                         else

src/core/Akka.Streams/TooManySubstreamsOpenException.cs

@@ -0,0 +1,22 @@
+//-----------------------------------------------------------------------
+// <copyright file="TooManySubstreamsOpenException.cs" company="Akka.NET Project">
+//     Copyright (C) 2009-2022 Lightbend Inc. <http://www.lightbend.com>
+//     Copyright (C) 2013-2022 .NET Foundation <https://github.com/akkadotnet/akka.net>
+// </copyright>
+//-----------------------------------------------------------------------
+
+using System;
+
+namespace Akka.Streams
+{
+    /// <summary>
+    /// This exception signals that the maximum number of substreams declared has been exceeded.
+    /// A finite limit is imposed so that memory usage is controlled.
+    /// </summary>
+    public class TooManySubstreamsOpenException : InvalidOperationException
+    {
+        public TooManySubstreamsOpenException() :
+            base("Cannot open a new substream as there are too many substreams open")
+        { }
+    }
+}