Fix modal dispose scroll reset

[Kevinjohn]

Description

Resets the body scrollbar state when disposing a shown modal.

Motivation & Context

Showing a modal adds modal-open to the body and hides body overflow. If dispose() is called while the modal is still shown, the instance is removed but the body scroll lock is left behind.

Expected: disposing the shown modal clears the scroll lock it created.
Actual: disposing the shown modal leaves modal-open on the body and keeps body.style.overflow set to hidden.

This mirrors the normal hide cleanup for the dispose path and adds unit coverage for disposing a shown modal.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Actual screenshots

Before disposing the shown modal, the body remains scroll-locked:

After the fix, disposing clears modal-open and body overflow:

Related issues

None.

Checks

• git diff --check
• npm exec -- eslint --report-unused-disable-directives js/src/modal.js js/tests/unit/modal.spec.js
• npm run js-test-karma