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/ParserTests.cs

@@ -275,4 +275,15 @@ public class ParserTests
         var ex = await ParseExpectingErrorAsync(input);
         ex.ShouldBeOfType<ProtocolViolationException>();
     }
+
+    // Mirrors Go TestParsePubSizeOverflow: oversized decimal payload lengths
+    // must be rejected during PUB argument parsing.
+    // Reference: golang/nats-server/server/parser_test.go TestParsePubSizeOverflow
+    [Fact]
+    public async Task Parse_pub_size_overflow_fails()
+    {
+        var ex = await ParseExpectingErrorAsync("PUB foo 1234567890\r\n");
+        ex.ShouldBeOfType<ProtocolViolationException>();
+        ex.Message.ShouldContain("Invalid payload size");
+    }
 }