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assert: fix NotSubset error messages using % v instead of %q (fixes 1800)

assert: fix NotSubset error messages using % v instead of %q (fixes 1800): a task in repo2rlenv-v083-pr diff (Harbor dataset). Title: assert: fix NotSubset error messages using % v instead of %q

The task

**Title:** assert: fix NotSubset error messages using %#v instead of %q

Part of AdithyaSK/repo2rlenv-v083-pr_diff.