feat: execute post-baseline jetstream parity plan

tests/NATS.Server.Tests/JetStreamStoreExpiryParityTests.cs

@@ -0,0 +1,24 @@
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.Tests;
+
+public class JetStreamStoreExpiryParityTests
+{
+    [Fact]
+    public async Task File_store_prunes_expired_messages_using_max_age_policy()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), $"nats-js-filestore-expiry-{Guid.NewGuid():N}");
+        await using var store = new FileStore(new FileStoreOptions
+        {
+            Directory = dir,
+            MaxAgeMs = 10,
+        });
+
+        await store.AppendAsync("orders.created", "old"u8.ToArray(), default);
+        await Task.Delay(20);
+        await store.AppendAsync("orders.created", "new"u8.ToArray(), default);
+
+        var state = await store.GetStateAsync(default);
+        state.Messages.ShouldBe((ulong)1);
+    }
+}