Multiple Event Grid system topics on the same source not allowed but not caught by WhatIf

**Describe the bug**
Multiple Event Grid system topics on the same source not allowed. If I write a bicep file with 2 topics point to the same source, WhatIf doesn't error out, but I'll get an error when trying to actually deploy it.

**To Reproduce**

Sample Bicep file
```
param storageAccountType string = 'Standard_LRS'
param location string = resourceGroup().location
param storageAccountName string = 'dfgsdfgsthrtwesdfg' // Change to other random string
param containerName string = 'testcontainer'

param eventGridSystemTopicName1 string = 'topic1'
param eventGridSystemTopicName2 string = 'topic2'

resource sa 'Microsoft.Storage/storageAccounts@2025-01-01' = {
name: storageAccountName
location: location
sku: {
name: storageAccountType
}
kind: 'StorageV2'
properties: {}
}

resource blob 'Microsoft.Storage/storageAccounts/blobServices@2025-01-01' = {
parent: sa
name: 'default'
}

resource container 'Microsoft.Storage/storageAccounts/blobServices/containers@2025-01-01' = {
parent: blob
name: containerName
}

resource eventGridSystemTopic1 'Microsoft.EventGrid/systemTopics@2023-12-15-preview' = {
name: eventGridSystemTopicName1
location: location
properties: {
source: sa.id
topicType: 'Microsoft.Storage.StorageAccounts'
}
}

resource eventGridSystemTopic2 'Microsoft.EventGrid/systemTopics@2023-12-15-preview' = {
name: eventGridSystemTopicName2
location: location
properties: {
source: sa.id
topicType: 'Microsoft.Storage.StorageAccounts'
}
}
```

**Expected behavior**
WhatIf to error out

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

**Client** CLI and GitHub action

Start by running the supplied Bicep reproduction through the CLI What-If flow, then compare it with an actual deployment. Trace how the GitHub Action and CLI report duplicate Event Grid system topics for one source. Done means What-If rejects the two-topic configuration with an actionable error before deployment.

Written by the indexing model from the issue text.