feat(p7-02): fill opts_test.go stubs — ServerOptionsTests

Write 3 unit tests portable without a running server:
- ListenMonitoringDefault (T:2524): SetBaselineOptions propagates Host → HttpHost
- GetStorageSize (T:2576): StorageSizeJsonConverter.Parse K/M/G/T suffixes
- ClusterNameAndGatewayNameConflict (T:2571): ValidateOptions returns ErrClusterNameConfigConflict

Mark 74 opts_test.go stubs deferred: tests require either the NATS
conf-format parser (not yet ported), a running server (RunServer/NewServer),
or CLI flag-parsing infrastructure (ConfigureOptions).

Fix StorageSizeJsonConverter.Parse to return 0 for empty input,
matching Go getStorageSize("") == (0, nil).

Total unit tests: 638 passing.

dotnet/src/ZB.MOM.NatsNet.Server/Config/NatsJsonConverters.cs

@@ -152,6 +152,8 @@ public sealed class StorageSizeJsonConverter : JsonConverter<long>
 
     public static long Parse(string s)
     {
+        // Mirrors Go getStorageSize: empty string returns 0 with no error.
+        if (string.IsNullOrWhiteSpace(s)) return 0;
         if (long.TryParse(s, out var n)) return n;
         var m = Pattern.Match(s.Trim());
         if (!m.Success) throw new FormatException($"Invalid storage size: \"{s}\"");