[BUG] 200 response for PartnerTopics PATCH in EventGrid missing body schema

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/06f69b8b78d525c5e47f81f50e2cb1d21b7dd783/specification/eventgrid/resource-manager/Microsoft.EventGrid/EventGrid/PartnerTopic.tsp#L56

### API Spec version

2025-04-01-preview

### Describe the bug

The atual 200 response of
`/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.EventGrid/partnerTopics/{partnerTopicName}`
has body schema, which is inconsistent with current definition.

### Expected behavior

Response definition has body schema

### Actual behavior

Response definition without body schema

### Reproduction Steps

Operation: PartnerTopics_Update HTTP PATCH `/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.EventGrid/partnerTopics/{partnerTopicName}`

### Environment

_No response_

Open EventGrid/PartnerTopic.tsp at line 56 and inspect the PartnerTopics_Update PATCH operation. Compare the defined 200 response with the reported response shape, then verify the response includes its body schema and run the repository's relevant API specification checks if available.

Written by the indexing model from the issue text.