[BUG] Gateway agentpool is configured with invalid node count 1, exceeding max node count 2

[JoeyC-Dev]

Describe the bug
1 node should be allowed when set the prefix with /31. But it says, "1 node is not allowed because it exceeds the max number 2".

To Reproduce
Steps to reproduce the behavior:

az extension add -n aks-preview

# Basic parameter
ranNum=$(echo $RANDOM)
rG=aks-staticegressgw-${ranNum}
aks=aks-staticegressgw-${ranNum}
location=southeastasia
k8sVer=1.31

az group create -n ${rG} -l ${location} -o none

# Create AKS
az aks create -n ${aks} -g ${rG} \
    --no-ssh-key \
    --nodepool-name agentpool \
    --node-os-upgrade-channel None \
    --node-count 1 --kubernetes-version ${k8sVer} \
    --node-vm-size Standard_A4_v2 \
    --network-plugin azure \
    --enable-static-egress-gateway

# Bug: Create a Gateway Node pool
az aks nodepool add --cluster-name ${aks} -g ${rG} -n gatewaypool \
--mode gateway --node-count 1 --gateway-prefix-size 31

Output:

(StaticEgressGatewayInvalidGatewayNodeCount) Gateway agentpool gatewaypool is configured with invalid node count 1, exceeding max node count 2 according to public IPPrefix size /31 or fewer than 2.
Code: StaticEgressGatewayInvalidGatewayNodeCount
Message: Gateway agentpool gatewaypool is configured with invalid node count 1, exceeding max node count 2 according to public IPPrefix size /31 or fewer than 2.

Expected behavior
1 node should be allowed when setting gateway prefix with 31.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• Kubernetes version [e.g. 1.24.3] 1.31.1
• Browser: Cloud Shell
• CLI info:

{
 "azure-cli": "2.65.0",
 "azure-cli-core": "2.65.0",
 "azure-cli-telemetry": "1.1.0",
 "extensions": {
   "ai-examples": "0.2.5",
   "aks-preview": "9.0.0b7",
   "ml": "2.30.1",
   "ssh": "2.0.5"
 }
}

[JoeyC-Dev]

I just realise that "or fewer than 2" at the end of the output.
Close as intended.