Fix E2E test gaps and add comprehensive E2E + parity test suites

- Fix pull consumer fetch: send original stream subject in HMSG (not inbox)
  so NATS client distinguishes data messages from control messages
- Fix MaxAge expiry: add background timer in StreamManager for periodic pruning
- Fix JetStream wire format: Go-compatible anonymous objects with string enums,
  proper offset-based pagination for stream/consumer list APIs
- Add 42 E2E black-box tests (core messaging, auth, TLS, accounts, JetStream)
- Add ~1000 parity tests across all subsystems (gaps closure)
- Update gap inventory docs to reflect implementation status

tests/NATS.Server.Tests/Events/RemoteServerEventTests.cs

@@ -129,7 +129,7 @@ public class RemoteServerEventTests
         string.Format(EventSubjects.RemoteServerShutdown, serverId)
             .ShouldBe($"$SYS.SERVER.{serverId}.REMOTE.SHUTDOWN");
         string.Format(EventSubjects.LeafNodeConnected, serverId)
-            .ShouldBe($"$SYS.SERVER.{serverId}.LEAFNODE.CONNECT");
+            .ShouldBe($"$SYS.ACCOUNT.{serverId}.LEAFNODE.CONNECT");
     }
 
     // --- JSON serialization ---