🧪 Add test coverage for secret scanner hook

[google-labs-jules]

🎯 What: Adds comprehensive test coverage for hooks/secret_scanner.py to test secret detection logic.
📊 Coverage: Tests is_env_file, extract_content, and multiple positive/negative scenarios for scan_content (including rules like eth_private_key, mnemonic_phrase, aws_key, api_key_assignment, and jwt_token).
✨ Result: Provides a robust safety net to ensure no regressions are introduced into the secret scanning hook logic when future refactoring occurs.

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PR created automatically by Jules for task 16514947616525273861 started by @zknpr

[google-labs-jules]

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