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

src/NATS.Server/Protocol/NatsParser.cs

@@ -1,5 +1,6 @@
 using System.Buffers;
 using System.Text;
+using System.Text.Json;
 using Microsoft.Extensions.Logging;
 
 namespace NATS.Server.Protocol;
@@ -80,7 +81,14 @@ public sealed class NatsParser
 
         // Control line size check
         if (line.Length > NatsProtocol.MaxControlLineSize)
-            throw new ProtocolViolationException("Maximum control line exceeded");
+        {
+            var snippetLength = (int)Math.Min(line.Length, NatsProtocol.MaxControlLineSnippetSize);
+            var snippetBytes = new byte[snippetLength];
+            line.Slice(0, snippetLength).CopyTo(snippetBytes);
+            var snippet = ProtoSnippet(0, NatsProtocol.MaxControlLineSnippetSize, snippetBytes);
+            throw new ProtocolViolationException(
+                $"Maximum control line exceeded (max={NatsProtocol.MaxControlLineSize}, len={line.Length}, snip={snippet}...)");
+        }
 
         // Get line as contiguous span
         Span<byte> lineSpan = stackalloc byte[(int)line.Length];
@@ -95,7 +103,7 @@ public sealed class NatsParser
                 return false;
             }
 
-            throw new ProtocolViolationException("Unknown protocol operation");
+            throw new ProtocolViolationException($"Unknown protocol operation: {ProtoSnippet(lineSpan)}");
         }
 
         byte b0 = (byte)(lineSpan[0] | 0x20); // lowercase
@@ -192,9 +200,29 @@ public sealed class NatsParser
                 return true;
         }
 
-        throw new ProtocolViolationException("Unknown protocol operation");
+        throw new ProtocolViolationException($"Unknown protocol operation: {ProtoSnippet(lineSpan)}");
     }
 
+    // Go reference: parser.go protoSnippet(start, max, buf).
+    internal static string ProtoSnippet(int start, int max, ReadOnlySpan<byte> buffer)
+    {
+        if (start >= buffer.Length)
+            return "\"\"";
+
+        var stop = start + max;
+        if (stop > buffer.Length)
+            stop = buffer.Length - 1;
+
+        if (stop <= start)
+            return "\"\"";
+
+        var slice = buffer[start..stop];
+        return JsonSerializer.Serialize(Encoding.ASCII.GetString(slice));
+    }
+
+    internal static string ProtoSnippet(ReadOnlySpan<byte> buffer) =>
+        ProtoSnippet(0, NatsProtocol.ProtoSnippetSize, buffer);
+
     private bool ParsePub(
         Span<byte> line,
         ref ReadOnlySequence<byte> buffer,