feat: port session 11 — Accounts & Directory JWT Store

- Account: full Account class (200 features) with subject mappings,
  connection counting, export/import checks, expiration timers
- DirJwtStore: directory-based JWT storage with sharding and expiry
- AccountResolver: IAccountResolver, MemoryAccountResolver,
  UrlAccountResolver, DirAccountResolver, CacheDirAccountResolver
- AccountTypes: all supporting types (AccountLimits, SConns, ExportMap,
  ImportMap, ServiceExport, StreamExport, ServiceLatency, etc.)
- 34 unit tests (599 total), 234 features complete (IDs 150-349, 793-826)

dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Accounts.cs

@@ -671,8 +671,15 @@ public sealed partial class NatsServer
     {
         if (_accResolver == null)
             return (string.Empty, ServerErrors.ErrNoAccountResolver);
-        var (jwt, err) = _accResolver.Fetch(name);
-        return (jwt, err);
+        try
+        {
+            var jwt = _accResolver.FetchAsync(name).GetAwaiter().GetResult();
+            return (jwt, null);
+        }
+        catch (Exception ex)
+        {
+            return (string.Empty, ex);
+        }
     }
 
     /// <summary>