15-private-network-standard-agent-setup - Failing with existing VNET during template validation

15-private-network-standard-agent-setup bicep is failing with the following error message. Existing VNET + Subnets are being used. This is error during template validation. The deployment does not start.

`"submissionTimestamp": "2025-07-31T11:35:58Z",
"subscriptionId": "YYYYYYYYY",
"tenantId": "YYYYYYYYY",
"properties": {
"statusCode": "BadRequest",
"serviceRequestId": null,
"statusMessage": "{"error":{"code":"MultipleErrorsOccurred","message":"Multiple error occurred: BadRequest,BadRequest. Please see details.","details":[{"code":"InvalidTemplate","target":"/subscriptions/YYYYYYYYY/resourceGroups/YYYYYYYYY-ai-d/providers/Microsoft.Resources/deployments/sbn-YYY-YYYYY-foundry-agent-d","message":"Deployment template validation failed: 'The template resource '/sbn-YYY-YYYYY-foundry-agent-d' for type 'Microsoft.Network/virtualNetworks/subnets' at line '1' and column '1288' has incorrect segment lengths. A nested resource type must have identical number of segments as its resource name. A root resource type must have segment length one greater than its resource name. Please see https://aka.ms/arm-syntax-resources for usage details.'.","additionalInfo":[{"type":"TemplateViolation",

Start with the Bicep sample at samples/microsoft/infrastructure-setup/15-private-network-standard-agent-setup and inspect how the existing VNET and subnets are referenced during template validation. Reproduce validation with existing VNET and subnet inputs; done means the template validates without the incorrect segment-length error and deployment can start.