feat: enforce account jetstream limits and jwt tiers

src/NATS.Server/Auth/Jwt/AccountClaims.cs

@@ -47,6 +47,10 @@ public sealed class AccountNats
     [JsonPropertyName("limits")]
     public AccountLimits? Limits { get; set; }
 
+    /// <summary>JetStream entitlement limits/tier for this account.</summary>
+    [JsonPropertyName("jetstream")]
+    public AccountJetStreamLimits? JetStream { get; set; }
+
     /// <summary>NKey public keys authorized to sign user JWTs for this account.</summary>
     [JsonPropertyName("signing_keys")]
     public string[]? SigningKeys { get; set; }
@@ -92,3 +96,12 @@ public sealed class AccountLimits
     [JsonPropertyName("data")]
     public long MaxData { get; set; }
 }
+
+public sealed class AccountJetStreamLimits
+{
+    [JsonPropertyName("max_streams")]
+    public int MaxStreams { get; set; }
+
+    [JsonPropertyName("tier")]
+    public string? Tier { get; set; }
+}