[Bug]: Modal overlays allow for page scroll when open

### Code of conduct

- [x] I agree to follow this project's code of conduct.

### Impacted component(s)

Overlay

### Library version

1.7.0

### Expected behavior

When a [modal overlay](https://opensource.adobe.com/spectrum-web-components/components/overlay/#trigger) is open, it does not allow for page scroll.

### Actual behavior

When a [modal overlay](https://opensource.adobe.com/spectrum-web-components/components/overlay/#trigger) is open, it lets the page scroll.

### Screenshots

https://github.com/user-attachments/assets/94daf6a7-ea4a-4d86-ba27-c0b4aee54834

### What browsers are you seeing the problem in?

Microsoft Edge, Firefox, Chrome, Safari

### How can we reproduce this issue?

1. Go to https://opensource.adobe.com/spectrum-web-components/components/overlay/#trigger
2. Click on the button "Open modal"
3. Try to scroll the page
4. Validate the page actually scrolled when the expected behavior is it shouldn't have

### Sample code or abstract reproduction which illustrates the problem

_No response_

### Severity

SEV 3

### Logs taken while reproducing problem

_No response_

### Would you like to track this issue in Jira?

- [x] Yes, please tell me the ticket number!

Start with the Overlay component's trigger example at https://opensource.adobe.com/spectrum-web-components/components/overlay/#trigger and reproduce the issue in Chrome, Firefox, Edge, or Safari. Confirm that opening a modal prevents page scrolling while it remains open, and verify that normal page scrolling returns after it closes.

Written by the indexing model from the issue text.