Replace the bash pattern-substitution loop with grep -F -o piped to wc -l so the helper counts literal substring occurrences instead of treating the expected value as a glob pattern. The old implementation interpreted `[`, `]`, `*`, `?`, and `\` as pattern syntax, which made `assert_output_contains "['task.py', 'test']"` report 17 matches against an output that contained the string exactly once - the inner characters were being matched as a character class. assert_output_not_contains delegates to assert_output_contains and is fixed transitively.
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