fleet: require a minimum version of the peer at the handshake [CI]

MoShiro__MinAgentVersion makes the backend refuse agents below it at hello (fleet_version_required); fleet.min_server_version or MOSHIRO_MIN_SERVER_VERSION makes the agent drop a backend below it. Either side waits five seconds and shakes hands again, because the other one may be updating.

backend/src/MoShiro.Backend.Gateway/GatewayServiceCollectionExtensions.cs

@@ -16,6 +16,7 @@ public static class GatewayServiceCollectionExtensions
     public static IServiceCollection AddMoShiroGateway(this IServiceCollection services)
     {
         services.AddSingleton<IAgentGateway, FleetGateway>();
+        services.AddSingleton<IFleetVersionPolicy, BackendVersionPolicy>();
         services.AddHostedService<ConnectionShutdown>();
         services.AddFleetEnrollment();
         services.AddFleetServer(options => options.NodeIdPrefix = AgentIds.Prefix)
@@ -44,3 +45,11 @@ public static class GatewayServiceCollectionExtensions
                 (context, parameters, ct) => services.GetRequiredService<TestService>().OnStatusAsync(context.Node.Id, parameters, ct));
     }
 }
+
+/// <summary>握手时的版本要求：告诉 Agent 面板的版本，并要求 Agent 不低于配置的最低版本。</summary>
+internal sealed class BackendVersionPolicy(BackendOptions options) : IFleetVersionPolicy
+{
+    public string? ServerVersion => BuildInfo.Version;
+
+    public string? MinNodeVersion => string.IsNullOrWhiteSpace(options.MinAgentVersion) ? null : options.MinAgentVersion;
+}