feat(quic): replace probe UDP data plane with QUIC datagrams

backend/agent/src/MoShiro.Agent/AgentConfig.cs

@@ -51,8 +51,15 @@ public sealed class ListenSection
     [JsonPropertyName("control_port")]
     public int ControlPort { get; set; } = 7443;
 
+    [JsonPropertyName("quic_ports")]
+    public List<int> QuicPorts { get; set; } = [7443];
+
+    /// <summary>Legacy config key accepted while deployments migrate to QUIC Datagram.</summary>
     [JsonPropertyName("udp_ports")]
-    public List<int> UdpPorts { get; set; } = [7443];
+    public List<int>? LegacyUdpPorts { get; set; }
+
+    [JsonIgnore]
+    public IReadOnlyList<int> EffectiveQuicPorts => LegacyUdpPorts ?? QuicPorts;
 
     [JsonPropertyName("tcp_ports")]
     public List<int> TcpPorts { get; set; } = [7444];