(aws-scheduler): deploy error when addding schedule group to existing schedule

### Describe the bug

Unable to attach a `ScheduleGroup` to an existing `Schedule`.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Version

_No response_

### Expected Behavior

Schedule groups should be able to be added/removed from schedules.

### Current Behavior

Cloudformation error when deploying the schedules:
```
Resource handler returned message: "Resource of type 'AWS::Scheduler::Schedule' with identifier '' was not found." (RequestToken: , HandlerErrorCode: NotFound)
```

### Reproduction Steps

1. Create a `Schedule` using the default schedule group (i.e. leave the `scheduleGroup` property empty)
2. Deploy
3. Create a `ScheduleGroup` and update the existing `Schedule` to pass `scheduleGroup`
4. Attempt to deploy, but it fails

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.1004.0

### Framework Version

_No response_

### Node.js Version

20

### OS

Ubuntu 24.04

### Language

TypeScript

### Language Version

_No response_

### Other information

We are an Internal Amazon team.

Start with the aws-scheduler Schedule and ScheduleGroup constructs and follow the listed reproduction steps. Inspect how changing scheduleGroup is represented during deployment; done means an existing default-group Schedule can be updated to a ScheduleGroup without the CloudFormation NotFound error.

Written by the indexing model from the issue text.