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

assert: fix NotSubset error messages using %% v instead of %%q: a task in repo2rlenv-commit-runtime-v2 (Harbor dataset). Title: assert.NotSubset produces garbled error messages for non-string element types

The task

**Title:** `assert.NotSubset` produces garbled error messages for non-string element types

Part of FineEnvs/repo2rlenv-commit-runtime.