Avoid fake secret literal in gate test

[Pigbibi]

Summary

• build the fake OpenAI-style key at runtime in the gate test
• keep the hardcoded-secret detection coverage while avoiding static secret-scanner false positives

Tests

• python3 -m pytest tests/test_gate_codex_app_review.py -q
• inline regex check: no sk-[A-Za-z0-9]{20,} literal remains in tests/test_gate_codex_app_review.py
• git diff --check

Note: full python3 -m pytest -q still hits existing collection errors for missing generated/modules unrelated to this one-line test change; the repository CI currently runs pytest with || true.

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