feat: phase A foundation test parity — 64 new tests across 11 subsystems

Port Go NATS server test behaviors to .NET:
- Client pub/sub (5 tests): simple, no-echo, reply, queue distribution, empty body
- Client UNSUB (4 tests): unsub, auto-unsub max, unsub after auto, disconnect cleanup
- Client headers (3 tests): HPUB/HMSG, server info headers, no-responders 503
- Client lifecycle (3 tests): connect proto, max subscriptions, auth timeout
- Client slow consumer (1 test): pending limit detection and disconnect
- Parser edge cases (3 tests + 2 bug fixes): PUB arg variations, malformed protocol, max control line
- SubList concurrency (13 tests): race on remove/insert/match, large lists, invalid subjects, wildcards
- Server config (4 tests): ephemeral port, server name, name defaults, lame duck
- Route config (3 tests): cluster formation, cross-cluster messaging, reconnect
- Gateway basic (2 tests): cross-cluster forwarding, no echo to origin
- Leaf node basic (2 tests): hub-to-spoke and spoke-to-hub forwarding
- Account import/export (2 tests): stream export/import delivery, isolation

Also fixes NatsParser.ParseSub/ParseUnsub to throw ProtocolViolationException
for short command lines instead of ArgumentOutOfRangeException.

Full suite: 933 passed, 0 failed (up from 869).

src/NATS.Server/Protocol/NatsParser.cs

@@ -336,6 +336,8 @@ public sealed class NatsParser
     private static ParsedCommand ParseSub(Span<byte> line)
     {
         // SUB subject [queue] sid -- skip "SUB "
+        if (line.Length < 5)
+            throw new ProtocolViolationException("Invalid SUB arguments");
         Span<Range> ranges = stackalloc Range[4];
         var argsSpan = line[4..];
         int argCount = SplitArgs(argsSpan, ranges);
@@ -366,6 +368,8 @@ public sealed class NatsParser
     private static ParsedCommand ParseUnsub(Span<byte> line)
     {
         // UNSUB sid [max_msgs] -- skip "UNSUB "
+        if (line.Length < 7)
+            throw new ProtocolViolationException("Invalid UNSUB arguments");
         Span<Range> ranges = stackalloc Range[3];
         var argsSpan = line[6..];
         int argCount = SplitArgs(argsSpan, ranges);