fix: stabilize flaky server API and spread gate tests

Serialize agent WebSocket writes (ping + JSON), mutex-protect spreadGateClock,
and harden deploy-plan spread route hint setup under parallel test runs.

server/internal/api/fleet_handler_test.go

@@ -92,6 +92,25 @@ func testAgentClientIP(agentID string) string {
 	return fmt.Sprintf("10.42.%d.%d", (sum%250)+1, ((sum/250)%250)+1)
 }
 
+func waitForHubAgents(t *testing.T, hub *WSHub, agentIDs ...string) {
+	t.Helper()
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		allConnected := true
+		for _, id := range agentIDs {
+			if !hub.isAgentConnected(id) {
+				allConnected = false
+				break
+			}
+		}
+		if allConnected {
+			return
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	t.Fatalf("agents not connected: %v", agentIDs)
+}
+
 func connectTestAgent(t *testing.T, hub *WSHub, agentID string) *websocket.Conn {
 	t.Helper()
 	srv := httptest.NewServer(http.HandlerFunc(hub.HandleAgentWS))