Description
This PR fixes an accessibility issue where CheckedPopupMenuItem was not properly announcing its checked state to screen readers.

Before
https://github.com/user-attachments/assets/b1785522-eea4-4fc3-9a9a-1872615838c0

After
Demo app: https://checked-0707.web.app/
https://github.com/user-attachments/assets/a8083c0b-11c8-4e93-bd71-842dfe5fa344

Issues Fixed
This PR fixes #171463.

Breaking Changes
None.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• I followed the [breaking change policy] and added [Data Driven Fixes] where supported.
• All existing and new tests are passing.