Add test cases to improve test coverage

[seonghobae]

This PR adds missing testthat coverage for icci and vuongtest, particularly targeting edge cases and basic nested models test coverage which wasn't fully hit before. It also fixes some DESCRIPTION issues (Author and Maintainer entries) to pass R CMD check.

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PR created automatically by Jules for task 16256056548421475400 started by @seonghobae

[google-labs-jules]

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[seonghobae]

Closing during org-wide PR backlog reduction. This looks like a low-priority automated maintenance PR (test coverage only), while the repository has a large backlog of security, conflict, and required-workflow work. Reopen or recreate a fresh PR if this change is still needed.

[google-labs-jules]

Closing during org-wide PR backlog reduction. This looks like a low-priority automated maintenance PR (test coverage only), while the repository has a large backlog of security, conflict, and required-workflow work. Reopen or recreate a fresh PR if this change is still needed.

Understood. Acknowledging that this PR is being closed as part of a backlog reduction and stopping work on this task.