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/fleet/src/Fleet.Server.Link/NodeConnection.Receive.cs

@@ -151,6 +151,20 @@ internal sealed partial class NodeConnection
             return;
         }
 
+        string? minVersion = _host.VersionPolicy?.MinNodeVersion;
+        if (NodeVersions.IsBelow(hello.NodeVersion, minVersion))
+        {
+            // 版本不够：不登记在线，告诉节点要求的版本，关闭连接。节点等几秒（它可能正在更新）再握手。
+            FleetLog.VersionTooOld(_host.Logger, _node!.Id.Value, hello.NodeVersion, minVersion!);
+            if (message.Id is not null)
+            {
+                await SendAsync(RpcMessage.Failure(message.Id, new FleetError(FleetErrors.VersionRequired, $"node version {hello.NodeVersion} is below the required {minVersion}")), ct);
+            }
+
+            await CloseAsync("version required");
+            return;
+        }
+
         long nowMs = _host.Time.GetUtcNow().ToUnixTimeMilliseconds();
         _session = _session! with
         {
@@ -169,7 +183,7 @@ internal sealed partial class NodeConnection
             await SendAsync(
                 RpcMessage.Success(
                     message.Id.Value,
-                    new HelloResult { ServerTimeMs = nowMs, ClockOffsetMs = hello.NowUtcMs - nowMs },
+                    new HelloResult { ServerTimeMs = nowMs, ClockOffsetMs = hello.NowUtcMs - nowMs, ServerVersion = _host.VersionPolicy?.ServerVersion },
                     FleetRpcContext.Default.HelloResult),
                 ct);
         }