LastUserActivityTimestamp should not be updated by automatic Health Checks

### Product Version

- [ ] Amazon SageMaker Studio Classic
- [X] Amazon SageMaker Studio
- [ ] It is not related to SageMaker Studio

### Product Category

Other

### Description

The LastUserActivityTimestamp element of the DescribeApp API cannot be used to determine when a user interacted with an app for the last time because SageMaker also updates it when health checks are performed ([as documented](https://docs.aws.amazon.com/sagemaker/latest/APIReference/API_DescribeApp.html#API_DescribeApp_ResponseSyntax)).

This element should not be updated on health checks since there is another dedicated field for this specific purpose (LastHealthCheckTimestamp).

### Other Details

_No response_

Start with the DescribeApp API documentation linked in the issue, comparing LastUserActivityTimestamp with LastHealthCheckTimestamp during automatic health checks. The issue is done when LastUserActivityTimestamp changes only after user interaction, while health checks update only LastHealthCheckTimestamp; no repository files or tests are identified.

Written by the indexing model from the issue text.