🧪 Add test for invalid JSON parsing in secret_scanner main

[google-labs-jules]

🎯 What: Added a missing unit test to hooks/secret_scanner.py to cover the scenario where sys.stdin.read() provides invalid JSON to the main() function.

📊 Coverage: The new test ensures that the main() function correctly captures json.JSONDecodeError or ValueError, logs an error using debug_log, and cleanly exits with sys.exit(0).

✨ Result: Test coverage is enhanced for error scenarios, preventing regressions in the hook's entry point exception handling logic.

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PR created automatically by Jules for task 6179579569845950508 started by @zknpr

[google-labs-jules]

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