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/WebSocket/WsUpgradeHelperParityBatch1Tests.cs

@@ -0,0 +1,66 @@
+using System.Text;
+using NATS.Server.WebSocket;
+
+namespace NATS.Server.Tests.WebSocket;
+
+public class WsUpgradeHelperParityBatch1Tests
+{
+    [Fact]
+    public void MakeChallengeKey_returns_base64_of_16_random_bytes()
+    {
+        var key = WsUpgrade.MakeChallengeKey();
+        var decoded = Convert.FromBase64String(key);
+
+        decoded.Length.ShouldBe(16);
+    }
+
+    [Fact]
+    public void Url_helpers_match_ws_and_wss_schemes()
+    {
+        WsUpgrade.IsWsUrl("ws://localhost:8080").ShouldBeTrue();
+        WsUpgrade.IsWsUrl("wss://localhost:8443").ShouldBeFalse();
+        WsUpgrade.IsWsUrl("http://localhost").ShouldBeFalse();
+
+        WsUpgrade.IsWssUrl("wss://localhost:8443").ShouldBeTrue();
+        WsUpgrade.IsWssUrl("ws://localhost:8080").ShouldBeFalse();
+        WsUpgrade.IsWssUrl("https://localhost").ShouldBeFalse();
+    }
+
+    [Fact]
+    public async Task RejectNoMaskingForTest_forces_no_masking_handshake_rejection()
+    {
+        var request = BuildValidRequest("/leafnode", "Nats-No-Masking: true\r\n");
+        using var input = new MemoryStream(Encoding.ASCII.GetBytes(request));
+        using var output = new MemoryStream();
+
+        try
+        {
+            WsUpgrade.RejectNoMaskingForTest = true;
+            var result = await WsUpgrade.TryUpgradeAsync(input, output, new WebSocketOptions { NoTls = true });
+
+            result.Success.ShouldBeFalse();
+            output.Position = 0;
+            var response = Encoding.ASCII.GetString(output.ToArray());
+            response.ShouldContain("400 Bad Request");
+            response.ShouldContain("invalid value for no-masking");
+        }
+        finally
+        {
+            WsUpgrade.RejectNoMaskingForTest = false;
+        }
+    }
+
+    private static string BuildValidRequest(string path = "/", string extraHeaders = "")
+    {
+        var sb = new StringBuilder();
+        sb.Append($"GET {path} HTTP/1.1\r\n");
+        sb.Append("Host: localhost:8080\r\n");
+        sb.Append("Upgrade: websocket\r\n");
+        sb.Append("Connection: Upgrade\r\n");
+        sb.Append("Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\n");
+        sb.Append("Sec-WebSocket-Version: 13\r\n");
+        sb.Append(extraHeaders);
+        sb.Append("\r\n");
+        return sb.ToString();
+    }
+}