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

NatsDotNet.slnx

@@ -5,5 +5,6 @@
   </Folder>
   <Folder Name="/tests/">
     <Project Path="tests/NATS.Server.Tests/NATS.Server.Tests.csproj" />
+    <Project Path="tests/NATS.E2E.Tests/NATS.E2E.Tests.csproj" />
   </Folder>
 </Solution>