feat: complete remaining jetstream parity implementation plan

tests/NATS.Server.Tests/JetStreamStreamMessageApiTests.cs

@@ -0,0 +1,20 @@
+namespace NATS.Server.Tests;
+
+public class JetStreamStreamMessageApiTests
+{
+    [Fact]
+    public async Task Stream_msg_get_delete_and_purge_change_state()
+    {
+        await using var fx = await JetStreamApiFixture.StartWithStreamAsync("ORDERS", "orders.*");
+        var ack = await fx.PublishAndGetAckAsync("orders.created", "1");
+
+        var get = await fx.RequestLocalAsync("$JS.API.STREAM.MSG.GET.ORDERS", $"{{\"seq\":{ack.Seq}}}");
+        get.StreamMessage.ShouldNotBeNull();
+
+        var del = await fx.RequestLocalAsync("$JS.API.STREAM.MSG.DELETE.ORDERS", $"{{\"seq\":{ack.Seq}}}");
+        del.Success.ShouldBeTrue();
+
+        var purge = await fx.RequestLocalAsync("$JS.API.STREAM.PURGE.ORDERS", "{}");
+        purge.Success.ShouldBeTrue();
+    }
+}