feat: complete remaining jetstream parity implementation plan

tests/NATS.Server.Tests/JetStreamPushConsumerContractTests.cs

@@ -0,0 +1,17 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamPushConsumerContractTests
+{
+    [Fact]
+    public async Task Ack_all_advances_floor_and_clears_pending_before_sequence()
+    {
+        await using var fx = await JetStreamApiFixture.StartWithAckAllConsumerAsync();
+        await fx.PublishManyAsync("orders.created", ["1", "2", "3"]);
+
+        var first = await fx.FetchAsync("ORDERS", "ACKALL", 3);
+        await fx.AckAllAsync("ORDERS", "ACKALL", first.Messages.Last().Sequence);
+
+        var pending = await fx.GetPendingCountAsync("ORDERS", "ACKALL");
+        pending.ShouldBe(0);
+    }
+}