Fix offcanvas dispose scroll reset

[Kevinjohn]

Description

Resets the body scrollbar state when disposing a shown offcanvas with scrolling disabled.

Motivation & Context

Showing an offcanvas with the default scroll: false hides body overflow. If dispose() is called while the offcanvas is still shown, the instance is removed but the body scroll lock is left behind.

Expected: disposing the shown offcanvas clears the scroll lock it created.
Actual: disposing the shown offcanvas leaves body.style.overflow set to hidden.

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

Related closed reports: #39910 and #36397.

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 offcanvas, body overflow remains locked:

After the fix, disposing clears the body overflow lock:

Related issues

Related: #39910, #36397.

Checks

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