[cupertino_ui] Migrate a snippet in CupertinoCheckbox's API doc to {@example} and add unit tests

[dkwingsmt]

This PR demonstrates how to migrate an inline API documentation snippet into a separate file and verify it with unit tests.

Steps

1. Create a new example file.
   • The file name should contain snippet.
   • The example file doesn't need a main function.
   • The widget body should contain the snippet, which is isolated by #region and #endregion directives.
   • If necessary, the file can have multiple regions. The region name doesn't have to be body.
2. Add a corresponding test file to ensure the snippet functions correctly.
3. Update the original documentation to use the new @example directive.
4. Remove the TODO.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²